How to build an OData Service with Olingo V4

Part 4: Navigation

Introduction

In the present tutorial, we will learn how to implement navigation between 2 Entity Types in an OData V4 service.

Note

The final source code can be found in the project git repository. A detailed description how to checkout the tutorials can be found here.
This tutorial can be found in subdirectory \samples\tutorials\p4_navigation

Disclaimer

Again, in the present tutorial, we will focus only on the relevant implementation, in order to keep the code small and simple. The sample code shouldn’t be reused for advanced scenarios.

Background

Say, we have an electronics shop and we have a lot of products which we’re selling and these products can be notebooks or monitors or organizers, which are the categories. We would have 3 requirements:

  1. We want to show a list of all our categories, then select one and display a list of all products that belong to this category, e.g. all monitors In terms of OData, this is called navigation
  2. From the list of our products, we want to choose one and display its category
  3. We want to navigate from a category to its products and perform a READ operation on one of them.

Example for navigating in a service

We open the Categories collection: http://localhost:8080/DemoService/DemoService.svc/Categories

CategoryCollection

We open the details of the first Category, the “Notebooks”-category: http://localhost:8080/DemoService/DemoService.svc/Categories(1)

CategoryEntity

In order to display all products that are notebooks, we can navigate from the selected category to its products: http://localhost:8080/DemoService/DemoService.svc/Categories(1)/Products

ProductsOfCategory

In the above example we’ve executed a one-to-many navigation.

As mentioned in the Background section, it is also required to navigate from a selected product to its category, which is a to-one relation, like: http://localhost:8080/DemoService/DemoService.svc/Products(1)/Category

And finally, it is possible to navigate to a list of products and directly access one of them, e.g. http://localhost:8080/DemoService/DemoService.svc/Categories(1)/Products(1)

All three cases are covered by the present tutorial.

Table of Contents

  1. Prerequisites
  2. Preparation
  3. Implementating the navigation
    1. Declare the Metadata
    2. Implement the to-many navigation
    3. Implement the to-one navigation
    4. Implement the to-many navigation with key access
  4. Run the implemented service
  5. Summary
  6. Links
  7. Appendix: code snippets

1. Prerequisites

Same prerequisites as in Tutorial Part 1: Read Entity Collection and Tutorial Part 2: Read Entity as well as basic knowledge about the concepts presented in both tutorials.


2. Preparation

Follow Tutorial Part 1: Read Entity Collection and Tutorial Part 2: Read Entity or as shortcut import Part 2: Read Entity, Read Property into your Eclipse workspace.

Afterwards do a Deploy and run: it should be working.


3. Implementing the navigation

In our sample scenario, we want to navigate from a product to its category and from a category to a list of products. In order to achieve this, we need to create a second Entity Type, "Category", and we need to specify Navigation Properties in both Entity Types. Our model looks as follows:

ODataModelNavigation

Note When designing the OData model, we could think of specifying a property “ProductCategory” in the entity type “Product”. E.g. a product with name “Brilliant flat and wide” would have the category “Monitors”. But this is not necessary, because that information can be obtained by navigating to the respective “Category”-entity using the navigation property. That way, we can keep the entity types lightweight, which is one of the intentions of OData.

3.1. Declare the metadata

In order to declare the metadata of our OData service, we open the class myservice.mynamespace.service.DemoEdmProvider_

3.1.1. Extend the Entity Type “Product”

In the previous tutorial we’ve already created the metadata for the “Product” entity type:

<EntityType Name="Product">
    <Key>
      <PropertyRef Name="ID" />
    </Key>
    <Property Name="ID" Type="Edm.Int32" Nullable="false" />
    <Property Name="Name" Type="Edm.String" Nullable="false" />
    <Property Name="Description" Type="Edm.String" Nullable="false" />
</EntityType>

Now we have to add a navigation property. That navigation property element has the following attributes:

Name The name of the navigation property is used as segment in the URI e.g. for the following URL: http://localhost:8080/DemoService/DemoService.svc/Products(1)/Category The segment “Category” is the name of the navigation property

Type Here we specify the Entity Type to which we’re navigating. e.g. for the following URL: http://localhost:8080/DemoService/DemoService.svc/Products(1)/Category we’re navigating to an entity which has the entity type “OData.Demo.Category” (we still have to create this entity type in this tutorial) Note that the fully qualified name has to be specified. Note that here we don’t specify a collection, so we have a to-one relationship.

Nullable Specifies if the navigation target is required. If we don’t specify it, then the default is assumed to be “true”. In our example we want to declare that every product must have a category, so we have to set it to “false”

Partner An optional attribute, used to define a bi-directional relationship. Specifies a path from the entity type (specified here) to the navigation property (defined there). In our example, we can navigate from product to category and from category to product

In our example, the metadata of our “Product” entity type looks as follows:

 <EntityType Name="Product">
   <Key>
     <PropertyRef Name="ID"/>
   </Key>
   <Property Name="ID" Type="Edm.Int32"/>
   <Property Name="Name" Type="Edm.String"/>
   <Property Name="Description" Type="Edm.String"/>
   <NavigationProperty Name="Category" Type="OData.Demo.Category" Nullable="false" Partner="Products"/>
 </EntityType>

Implementation-wise we have to create and configure an object of type CsdlNavigationProperty:

CsdlNavigationProperty navProp = new CsdlNavigationProperty()
                                    .setName("Category")
                                    .setType(ET_CATEGORY_FQN)
                                    .setNullable(false)
                                    .setPartner("Products");

Since an entity type can have multiple navigation properties, we have to put it into a list:

List<CsdlNavigationProperty> navPropList = new ArrayList<CsdlNavigationProperty>();
navPropList.add(navProp);

That list becomes relevant for the entity type that has been created earlier:

entityType.setNavigationProperties(navPropList);

There’s one more step to consider with respect to the navigation: the entity set. At runtime, we need to know how to implement the navigation, when an entity set is invoked. For this purpose, the OData specifies the NavigationPropertyBinding element, which is a child element of the entity set and should be defined for each navigation property. That NavigationPropertyBinding has the following attributes:

Path Here we specify the name of the corresponding navigation property. In our example, the navigation property that we’ve defined above is named “Category”

Target Here we specify the entity set where we’re navigating to. In our example it is the entity set “Categories” (which we will create below)

In our example, the definition of our “Products” entity set looks as follows:

<EntitySet Name="Products" EntityType="OData.Demo.Product">
  <NavigationPropertyBinding Path="Category" Target="Categories"/>
</EntitySet>

Code-wise, the getEntitySet method is extended as follows:

CsdlNavigationPropertyBinding navPropBinding = new CsdlNavigationPropertyBinding();
navPropBinding.setPath("Category"); // the path from entity type to navigation property
navPropBinding.setTarget("Categories"); //target entitySet, where the nav prop points to
List<CsdlNavigationPropertyBinding> navPropBindingList = new ArrayList<CsdlNavigationPropertyBinding>();
navPropBindingList.add(navPropBinding);
entitySet.setNavigationPropertyBindings(navPropBindingList);

3.1.2. Create the Entity Type “Category”

Now we have to create the second entity type, the “Category”. In order to keep our sample as simple as possible, we define only 2 properties, the “ID” and a “Name”. Since we want to be able to navigate from one given category (e.g. “Monitors”) to a list of products (e.g. all products that are monitors), we have to specify a navigation property in this entity type as well. Here, the navigation property has the following attributes:

Name In our example, we specify “Products”, in plural because we want to get multiple entities.

Type The “Type” attribute can be either an “entity type” or a “collection of entity types” In our example this time, we specify a “collection” of “OData.Demo.Product”

Nullable According to the OData specification (see odata.org), this attribute is not allowed for a collection A collection can be empty, but never null.

Partner In our example, we’re defining a bi-directional navigation, so here we specify “Category”, the name of the navigation property defined above.

In our example, the metadata of our “Category” entity type looks as follows:

<EntityType Name="Category">
  <Key>
    <PropertyRef Name="ID"/>
  </Key>
  <Property Name="ID" Type="Edm.Int32"/>
  <Property Name="Name" Type="Edm.String"/>
  <NavigationProperty Name="Products" Type="Collection(OData.Demo.Product)" Partner="Category"/>
</EntityType>

The code for the “Category” entity type:

if (entityTypeName.equals(ET_CATEGORY_FQN)){
    //create EntityType properties
    CsdlProperty id = new CsdlProperty()
                            .setName("ID")
                            .setType(EdmPrimitiveTypeKind.Int32.getFullQualifiedName());
    CsdlProperty name = new CsdlProperty()
                            .setName("Name")
                            .setType(EdmPrimitiveTypeKind.String.getFullQualifiedName());

    // create PropertyRef for Key element
    CsdlPropertyRef propertyRef = new CsdlPropertyRef();
    propertyRef.setName("ID");

    // navigation property: one-to-many
    CsdlNavigationProperty navProp = new CsdlNavigationProperty()
                            .setName("Products")
                            .setType(ET_PRODUCT_FQN)
                            .setCollection(true)
                            .setPartner("Category");
    List<CsdlNavigationProperty> navPropList = new ArrayList<CsdlNavigationProperty>();
    navPropList.add(navProp);

    // configure EntityType
    entityType = new CsdlEntityType();
    entityType.setName(ET_CATEGORY_NAME);
    entityType.setProperties(Arrays.asList(id, name));
    entityType.setKey(Arrays.asList(propertyRef));
    entityType.setNavigationProperties(navPropList);
}

The NavigationPropertyBinding element and its attributes for the entity set “Categories”:

Path In our example, the navigation property that we’ve defined above is named “Products”

Target In our example it is the entity set “Products”

In our example, the definition of our “Categories” entity set looks as follows:

<EntitySet Name="Categories" EntityType="OData.Demo.Category">
  <NavigationPropertyBinding Path="Products" Target="Products"/>
</EntitySet>

And the implementation in the getEntitySet method:

CsdlNavigationPropertyBinding navPropBinding = new CsdlNavigationPropertyBinding();
navPropBinding.setTarget("Products");//target entitySet, where the nav prop points to
navPropBinding.setPath("Products"); // the path from entity type to navigation property
List<CsdlNavigationPropertyBinding> navPropBindingList = new ArrayList<CsdlNavigationPropertyBinding>();
navPropBindingList.add(navPropBinding);
entitySet.setNavigationPropertyBindings(navPropBindingList);

3.2. Implement the to-many navigation

Let’s again have a look at our example, as described in the introduction section above. The user of our service invokes the “Categories” collection and chooses one “Category”. This is done with e.g. the following URL: http://localhost:8080/DemoService/DemoService.svc/Categories(1)

The returned response payload doesn’t contain any information about possible navigation. So the user has to check the metadata document, where he can see that the entity type “Category” defines one navigation property:

CategoryMetadata

This means, that he can append the navigation property name to his URL, which takes him to the set of “Products” that belong to the chosen “Category”: http://localhost:8080/DemoService/DemoService.svc/Categories(1)/Products

From the metadata we can see that the “Type” attribute defines a collection. This means that the implementation has to be done in the EntityCollectionProcessor, since we have to provide a collection of entities.

Open the class myservice.mynamespace.service.DemoEntityCollectionProcessor.java

There, the implementation for a “normal” read operation is already in place and we have to add the case when an entity collection is expected after navigation. Note that we want to keep our tutorial and our code simple, so we decide that only one step navigation is to be supported by our service. This means that we can navigate only once from one entity to another one. For example: Categories(1)/Products We don’t support navigation from one entity to an entity and then to another entity and so on For example: Categories(1)/Products(1)/Category

Based on this assumption, in our EntityCollectionProcessor, we can rely on the fact that the URI can have either one or two segments. This means: we can be called for the following kind of URLs:

Example URL for one sement: http://localhost:8080/DemoService/DemoService.svc/Categories

Example URL for two segments: http://localhost:8080/DemoService/DemoService.svc/Categories(1)/Products

As such, in our code we distinguish these 2 cases:

if(segmentCount == 1){
    // here the “normal” entity set is requested
}else if (segmentCount == 2){
    // this is reached in case of navigation: DemoService.svc/Categories(1)/Products
}else{
    // in our example, we don’t support URIs like Products(1)/Category/Products
    throw new ODataApplicationException("Not supported", HttpStatusCode.NOT_IMPLEMENTED.getStatusCode(),Locale.ENGLISH);
}

The segments of the URI are retrieved from the uriInfo parameter:

List<UriResource> resourceParts = uriInfo.getUriResourceParts();
int segmentCount = resourceParts.size();

In both cases, we have to retrieve the list of entities to be returned. For the first case, we have only one entitySet, so the implementation is straight forward:

EdmEntitySet startEdmEntitySet = uriResourceEntitySet.getEntitySet();
if(segmentCount == 1) {
    // 2nd: fetch the data from backend for this requested EntitySetName
    responseEntityCollection = storage.readEntitySetData(responseEdmEntitySet);
    responseEdmEntitySet = startEdmEntitySet; //there’s only one entity set
}

Now let’s focus on the second case, the navigation.

Our tasks are: 1. depending on the chosen key of the first segment, we have to compute which and how many entities exactly have to be returned. With other words, find the right data in the backend e.g. for the category “monitors”, we have to find the right products that are monitors 2. find out, which entity set has to be returned (can be products, categories, etc) This EdmEntitySet is required in order to properly build the context URL

The following sections explain how to do that.

3.2.1. Get the data for the response

Getting the data for the response is reylized in 2 steps:

A) get the data for the first URI segment in our example, we have to perform a read operation for retrieving the Category with ID 3, which is "Monitors"

B) get the data for the navigation in our example, we have to find the products that are monitors.

With respect to data, remember that we're using sample data that we create in our Storage class which represents our kind of database-mock.
On startup of our service, we initialize some sample products and categories. During initialization, there’s no assignment of products to its categories. In our sample code, we’re doing this when requested in a hard-coded method in our Storage class.

A) get the data for the first URI segment

In our example, the URL would be: http://localhost:8080/DemoService/DemoService.svc/Categories(3)/Products

For this example, we would have to retrieve the Category with ID=3.
The code looks like a normal READ operation:

List<UriParameter> keyPredicates = uriResourceEntitySet.getKeyPredicates();
Entity sourceEntity = storage.readEntityData(startEdmEntitySet, keyPredicates);

In our example, the result is an entity that represents the “Monitors” – category.

B) get the data for the navigation

Now we have to follow the navigation, based on the retrieved entity.
In our example, we have to retrieve all products that are monitors.

This is backend logic, so we can directly call a helper method in our Storage class:

responseEntityCollection = storage.getRelatedEntityCollection(sourceEntity, targetEntityType);

This helper method requires the source entity and returns the target collection. Additionally, the method needs the EdmEntityType that corresponds to the requested target. In our example, we pass the “Category” (i.e. "Monitors") as source entity and the navigation target entity type, which is “Product”. As a result, we get the desired “Products” collection, all products that are monitors.

After this step, we’re almost done, because we have the entity collection that our OData service returns in the response body. We only need to do some more hand work: the response entity collection has to be serialized and the serializer which is in charge of doing that has to be configured properly. For that we need the EdmEntitySet that corresponds to the response. Since it is different in case of navigation and non-navigation, we still need to retrieve it for the case of navigation.

3.2.2. Retrieve the EdmEntitySet for the response

First, we have to analyze the URI, and find out if the URI segment is used for navigation. As mentioned, in our simple example we assume that the second segment is used for navigation (in advanced services, a segment could as well be an action or function import, etc). The navigation URI segment can then be asked for the corresponding EdmNavigationProperty

UriResource lastSegment = resourceParts.get(1);
if(lastSegment instanceof UriResourceNavigation){
    UriResourceNavigation uriResourceNavigation = (UriResourceNavigation)lastSegment;
    EdmNavigationProperty edmNavigationProperty = uriResourceNavigation.getProperty();

The bad news is that the EdmNavigationProperty doesn’t know about the target EdmEntitySet. This is as per design, just check the metadata:

<EntityType Name="Category">
    ...
    <NavigationProperty Name="Products" Type="Collection(OData.Demo.Product)" Partner="Category"/>
</EntityType>

The navigation property is defined on entity-type-level and as such, it does know the target entity type. The target entity set is defined in the navigation property binding element on entity-set-level:

<EntitySet Name="Categories" EntityType="OData.Demo.Category">
    <NavigationPropertyBinding Path="Products" Target="Products"/>
</EntitySet>

This is where we get the information that we need.

For our implementation, this means: 1. we need the EdmEntitySet that corresponds to the first segment of the URI in our example: Categories 2. we need the navigation property that corresponds to the second segment of the URI in our example: Products

As shown below, from the source EdmEntitySet we get the binding target, based on the navigation property.

EdmBindingTarget edmBindingTarget = startEdmEntitySet.getRelatedBindingTarget(navPropName);
if(edmBindingTarget instanceof EdmEntitySet){
    navigationTargetEntitySet = (EdmEntitySet)edmBindingTarget;

This target is the entity set that we need.

We move the code into the utility method Util.getNavigationTargetEntitySet(startEdmEntitySet, edmNavigationProperty) Reason is that we'll need it again, later in this tutorial.

3.2.3 Remaining tasks

In the previous tutorials we’ve already learned what else has to be done: transform the retrieve data into an InputStream i.e. serialize the content. Furthermore, configure the response object, i.e. set the response body, the content type and the header.

The following snippet shows the implementation of the readEntityCollection(…) method.

public void readEntityCollection(ODataRequest request, ODataResponse response, UriInfo uriInfo, ContentType responseFormat)
                                throws ODataApplicationException, SerializerException {

EdmEntitySet responseEdmEntitySet = null; // for building ContextURL
EntityCollection responseEntityCollection = null; // for the response body

// 1st retrieve the requested EntitySet from the uriInfo
List<UriResource> resourceParts = uriInfo.getUriResourceParts();
int segmentCount = resourceParts.size();

UriResource uriResource = resourceParts.get(0); // the first segment is the EntitySet
if (! (uriResource instanceof UriResourceEntitySet)) {
    throw new ODataApplicationException("Only EntitySet is supported", HttpStatusCode.NOT_IMPLEMENTED.getStatusCode(),Locale.ROOT);
}

UriResourceEntitySet uriResourceEntitySet = (UriResourceEntitySet) uriResource;
EdmEntitySet startEdmEntitySet = uriResourceEntitySet.getEntitySet();

if(segmentCount == 1){ // this is the case for: DemoService/DemoService.svc/Categories
    responseEdmEntitySet = startEdmEntitySet; // first (and only) entitySet

    // 2nd: fetch the data from backend for this requested EntitySetName
    responseEntityCollection = storage.readEntitySetData(startEdmEntitySet);
}else if (segmentCount == 2){ //navigation: e.g. DemoService.svc/Categories(3)/Products
    UriResource lastSegment = resourceParts.get(1); // don't support more complex URIs
    if(lastSegment instanceof UriResourceNavigation){
        UriResourceNavigation uriResourceNavigation = (UriResourceNavigation)lastSegment;
        EdmNavigationProperty edmNavigationProperty = uriResourceNavigation.getProperty();
        EdmEntityType targetEntityType = edmNavigationProperty.getType();
        responseEdmEntitySet = Util.getNavigationTargetEntitySet(startEdmEntitySet, edmNavigationProperty);

        // 2nd: fetch the data from backend
        // first fetch the entity where the first segment of the URI points to
        // e.g. Categories(3)/Products first find the single entity: Category(3)
        List<UriParameter> keyPredicates = uriResourceEntitySet.getKeyPredicates();
        Entity sourceEntity = storage.readEntityData(startEdmEntitySet, keyPredicates);
        // error handling for e.g.  DemoService.svc/Categories(99)/Products
        if(sourceEntity == null) {
            throw new ODataApplicationException("Entity not found.", HttpStatusCode.NOT_FOUND.getStatusCode(), Locale.ROOT);
        }
        // then fetch the entity collection where the entity navigates to
        responseEntityCollection = storage.getRelatedEntityCollection(sourceEntity, targetEntityType);
    }
}else{ // this would be the case for e.g. Products(1)/Category/Products
    throw new ODataApplicationException("Not supported", HttpStatusCode.NOT_IMPLEMENTED.getStatusCode(),Locale.ROOT);
}
    // 3rd: create and configure a serializer
    ContextURL contextUrl = ContextURL.with().entitySet(responseEdmEntitySet).build();
    final String id = request.getRawBaseUri() + "/" + responseEdmEntitySet.getName();
    EntityCollectionSerializerOptions opts = EntityCollectionSerializerOptions.with().contextURL(contextUrl).id(id).build();
    EdmEntityType edmEntityType = responseEdmEntitySet.getEntityType();

    ODataSerializer serializer = odata.createSerializer(responseFormat);
    SerializerResult serializerResult = serializer.entityCollection(this.srvMetadata, edmEntityType, responseEntityCollection, opts);

    // 4th: configure the response object: set the body, headers and status code
    response.setContent(serializerResult.getContent());
    response.setStatusCode(HttpStatusCode.OK.getStatusCode());
    response.setHeader(HttpHeader.CONTENT_TYPE, responseFormat.toContentTypeString());   
}

3.3. Implement the to-one navigation

As for the to-one navigation, it is the case if the navigation target is a single entity, not a collection. In our example, the following URL represents a to-one navigation: http://localhost:8080/DemoService/DemoService.svc/Products(1)/Category

The user of our service has chosen a product and wants to know to which category it belongs. He can find it out by following the navigation property. As per design, a product can only belong to one category (obviously, a product can only be a Notebook or a Monitor, not both). Therefore in our service, we’ve defined a navigation property that is not of type collection:

<NavigationProperty
    Name="Category"
    Type="OData.Demo.Category"
    Nullable="false"
    Partner="Products"/>

So when the user follows the navigation property in order to display the product category, he expects a response that contains only one entry. This means that we have to do the implementation in the EntityProcessor.

Open the class myservice.mynamespace.service.DemoEntityProcessor.java

As usual, we first have to analyze the URI. Just like we did in the EntityCollectionProcessor, we have to distinguish between navigation and “normal” read of an entity:

if(segmentCount == 1){
    // in case of directly adressing of an entity
}else if (segmentCount == 2){
    // this is reached in case of navigation
}

In the following section, we will focus on the navigation case only. In our example, our task is to find the category of a chosen product. Again, we have to first fetch the chosen product (first URI segment) from our database-mock and in a second step, we have to ask our database-mock for the corresponding category. This final entity is then serialized and set as response body for the readEntity() method, which we’re implementing.

A) get the data for the first URI segment

In our example, we have to perform a read operation for retrieving the product with ID 1: http://localhost:8080/DemoService/DemoService.svc/Products(1)/Category

The code is the same like in the previous chapter (to-many navigation):

List<UriParameter> keyPredicates = uriResourceEntitySet.getKeyPredicates();
Entity sourceEntity = storage.readEntityData(startEdmEntitySet, keyPredicates);

B) get the data for the navigation

Now we have to follow the navigation, based on the retrieved entity. In our example, we have to find the category corresponding to the chosen product. Therefore, we invoke our helper method and pass the source Entity (Product) and the required target entity type (Category). The method will find the category which is related to the chosen product.

responseEntity = storage.getRelatedEntity(sourceEntity, responseEdmEntityType);

Before we can serialize the responseEntity, we have to retrieve the EdmEntitySet that corresponds to the response entity, because we need it for building the ContextURL.

The procedure is the same like in the chapter above, where we treated the to-many navigation:

EdmNavigationProperty edmNavigationProperty = uriResourceNavigation.getProperty();
responseEdmEntityType = edmNavigationProperty.getType();
responseEdmEntitySet = Util.getNavigationTargetEntitySet(startEdmEntitySet, edmNavigationProperty);

In our example, the value of the variable responseEdmEntitySet will be “Categories” and it will be used for building the contextURL, which will look as follows:

"$metadata#Categories/$entity"

3.4. Implement the to-many navigation with key access

"Navigation with key access" means that we have a to-many navigation, like navigating from a chosen category to the list of corresponding products: http://localhost:8080/DemoService/DemoService.svc/Categories(3)/Products

but in addition, we want to read only one of the collected products, which is directly addressed by its key: http://localhost:8080/DemoService/DemoService.svc/Categories(3)/Products(5)

From this URL, we can assume that the EntityProcessor is the relevant place to handle this request in our code.

The steps to find the requested entity are:

  1. Do a read operation for the first segment (same as in the previous chapter) In our example, this would be read entity for: /Categories(3)
  2. Follow the navigation to get the collection of the second segment In our example, this would be get the entity collection for: /Categories(3)/Products
  3. Pick the requested entity from the collection In our example, retrieve the product with ID=5, which is contained in the collection /Categories(3)/Products(5)

We can assume, that our database-mock is able to perform step 2 and 3 together.

In our class myservice.mynamespace.service.DemoEntityProcessor.java, we’ve already added the navigation capability for to-one navigation. How can we find out that we aren’t called for a to-one navigation, but instead, we’re responding to a to-many navigation with key access? The difference is the “key predicate”. The necessary info about it can be obtained from the URI segment. In the first chapter, we’ve already learned that there’s a special interface responsible for navigation segments, the org.apache.olingo.server.api.uri.UriResourceNavigation It also provides a method getKeyPredicates() We can make use of it in order to distinguish between “to-one navigation” and “navigation with key access”. If the call to

List<UriParmeter> navKeyPredicates = uriResourceNavigation.getKeyPredicates();

returns an empty list, then we can assume that our OData service has been called for a “to-one navigation”. This to-one navigation has been explained in the chapter 3.3. above. If the service request is like /Categories(3)/Products(5) then the method getKeyPredicates() will return a list of with one element that contains ID=5

In our implementation of the EntityProcessor, we add the following code:

List<UriParameter> navKeyPredicates = uriResourceNavigation.getKeyPredicates();
if(navKeyPredicates.isEmpty()){
    responseEntity = storage.getRelatedEntity(sourceEntity, responseEdmEntityType);
}else{
    responseEntity = storage.getRelatedEntity(sourceEntity, responseEdmEntityType, navKeyPredicates);
}

We get the key predicates for the navigation segment. Then we check if returned list is empty. If yes, we use the line that we implemented in chapter 3.3. If not, we have to create a new helper method that uses the key predicates for retrieving the desired entity. The new helper method

responseEntity = storage.getRelatedEntity(sourceEntity, responseEdmEntityType, navKeyPredicates);

will take care of getting the collection of products (responseEntityType) that are in scope of the chosen category (sourceEntity) and will then pick the requested product, based on the given key (navKeyPredicates).

One last thing to consider:
As we mentioned above, the user of our service is expected to specify a key of a product that is contained in the collection of products (e.g. /Categories(3)/Products) that is addressed by e.g. /Categories(3)/Products(5).

But he might specify a product ID that is existing, but not valid for the addressed navigation e.g. /Categories(3)/Products(1).

With other words: it is not valid to navigate from category "Monitors" to a product like "Notebook Basic 15"

If this is the case, we have to throw an appropriate exception. However, in our simple example we’re satisfied with simply checking if an entity was found at all:

if(responseEntity == null) {
    throw new ODataApplicationException("Nothing found.", HttpStatusCode.NOT_FOUND.getStatusCode(), Locale.ROOT);
}

Note When implementing this navigation for the first time, our first intention might have been: Let’s just ignore the first segment and simply do a read operation for /Products(5) Which would mean, from the list of all products, pick the one with ID=5 Why not? The answer is that we cannot assume that the requested Product is automatically belonging to the specified Category. E.g. in our example, the following URI should throw an error: Categories(3)/Products(1) As we know, Categories(3) is “Monitors” and Product(1) is a “Notebook”

That’s it. We don’t need to do an additional effort to retrieve the EdmEntitySet for the ContextURL, because this has already been implemented in our DemoEntityProcessor in the context of the previous chapter 3.3.

So now we can finally have a look at the full implementation of the readEntity() method, the covers both the cases of chapter 3.3. and 3.4.

public void readEntity(ODataRequest request, ODataResponse response, UriInfo uriInfo, ContentType responseFormat)
            throws ODataApplicationException, SerializerException {


    EdmEntityType responseEdmEntityType = null; // we'll need this to build the ContextURL
    Entity responseEntity = null; // required for serialization of the response body
    EdmEntitySet responseEdmEntitySet = null; // we need this for building the contextUrl

    // 1st step: retrieve the requested Entity:
    // can be "normal" read operation, or navigation (to-one)
    List<UriResource> resourceParts = uriInfo.getUriResourceParts();
    int segmentCount = resourceParts.size();

    UriResource uriResource = resourceParts.get(0);
    if (! (uriResource instanceof UriResourceEntitySet)) {
        throw new ODataApplicationException("Only EntitySet is supported", HttpStatusCode.NOT_IMPLEMENTED.getStatusCode(),
                                                Locale.ROOT);
    }

    UriResourceEntitySet uriResourceEntitySet = (UriResourceEntitySet) uriResource;
    EdmEntitySet startEdmEntitySet = uriResourceEntitySet.getEntitySet();

    // Analyze the URI segments
    if(segmentCount == 1){  // no navigation
        responseEdmEntityType = startEdmEntitySet.getEntityType();
        responseEdmEntitySet = startEdmEntitySet; // since we have only one segment

        // 2. step: retrieve the data from backend
        List<UriParameter> keyPredicates = uriResourceEntitySet.getKeyPredicates();
        responseEntity = storage.readEntityData(startEdmEntitySet, keyPredicates);
    } else if (segmentCount == 2){ //navigation
        UriResource navSegment = resourceParts.get(1);
        if(navSegment instanceof UriResourceNavigation){
            UriResourceNavigation uriResourceNavigation = (UriResourceNavigation) navSegment;
            EdmNavigationProperty edmNavigationProperty = uriResourceNavigation.getProperty();
            responseEdmEntityType = edmNavigationProperty.getType();
            responseEdmEntitySet = Util.getNavigationTargetEntitySet(startEdmEntitySet, edmNavigationProperty);

            // 2nd: fetch the data from backend.
            // for:  Products(1)/Category  we have to find the correct Category entity
            List<UriParameter> keyPredicates = uriResourceEntitySet.getKeyPredicates();
            // e.g. for Products(1)/Category we have to find first the Products(1)
            Entity sourceEntity = storage.readEntityData(startEdmEntitySet, keyPredicates);

            // now we have to check if the navigation is
            // a) to-one: e.g. Products(1)/Category
            // b) to-many with key: e.g. Categories(3)/Products(5)
            List<UriParameter> navKeyPredicates = uriResourceNavigation.getKeyPredicates();

            if(navKeyPredicates.isEmpty()){
                // e.g. DemoService.svc/Products(1)/Category
                responseEntity = storage.getRelatedEntity(sourceEntity, responseEdmEntityType);
            }else{ // e.g. DemoService.svc/Categories(3)/Products(5)
                responseEntity = storage.getRelatedEntity(sourceEntity, responseEdmEntityType, navKeyPredicates);
            }
        }
    }else{
        // this would be the case for e.g. Products(1)/Category/Products(1)/Category
        throw new ODataApplicationException("Not supported", HttpStatusCode.NOT_IMPLEMENTED.getStatusCode(), Locale.ROOT);
    }

    if(responseEntity == null) {
        // this is the case for e.g. DemoService.svc/Categories(4) or
        // DemoService.svc/Categories(3)/Products(999)
        throw new ODataApplicationException("Nothing found.", HttpStatusCode.NOT_FOUND.getStatusCode(), Locale.ROOT);
    }

    // 3. serialize
    ContextURL contextUrl = ContextURL.with().entitySet(responseEdmEntitySet).suffix(Suffix.ENTITY).build();
    EntitySerializerOptions opts = EntitySerializerOptions.with().contextURL(contextUrl).build();

    ODataSerializer serializer = this.odata.createSerializer(responseFormat);
    SerializerResult serializerResult = serializer.entity(this.srvMetadata, responseEdmEntityType, responseEntity, opts);

    //4. configure the response object
    response.setContent(serializerResult.getContent());
    response.setStatusCode(HttpStatusCode.OK.getStatusCode());
    response.setHeader(HttpHeader.CONTENT_TYPE, responseFormat.toContentTypeString());
}

4. Run the implemented service

After building and deploying your service to your server, you can try the following URLs:


5. Summary

In this tutorial we have learned how to add navigation capabilities to an OData service. We’ve implemented the to-many and to-one relationship and also the READ access to one entity after a to-many navigation. We’ve restricted the navigation to two segments, no more than navigating from one entity to another one. The modification of relations has not been covered by this tutorial. Check the Links section for more OData V4 tutorials.


6. Links

Tutorials

Code and Repository

Further reading


7. Appendix: code snippets

When reaching the point where your OData service has to become productive and support complex scenarions, you’ll find the following code snippets useful.

7.1. Find the EdmEntitySet for the navigation target

public static EdmEntitySet getNavigationTargetEntitySet(final UriInfoResource uriInfo) throws ODataApplicationException {

    EdmEntitySet entitySet;
    final List<UriResource> resourcePaths = uriInfo.getUriResourceParts();

    // First must be entity set (hence function imports are not supported here).
    if (resourcePaths.get(0) instanceof UriResourceEntitySet) {
        entitySet = ((UriResourceEntitySet) resourcePaths.get(0)).getEntitySet();
    } else {
        throw new ODataApplicationException("Invalid resource type.",
                HttpStatusCode.NOT_IMPLEMENTED.getStatusCode(), Locale.ROOT);
    }

    int navigationCount = 0;
    while (entitySet != null
        && ++navigationCount < resourcePaths.size()
        && resourcePaths.get(navigationCount) instanceof UriResourceNavigation) {
        final UriResourceNavigation uriResourceNavigation = (UriResourceNavigation) resourcePaths.get(navigationCount);
        final EdmBindingTarget target = entitySet.getRelatedBindingTarget(uriResourceNavigation.getProperty().getName());
        if (target instanceof EdmEntitySet) {
            entitySet = (EdmEntitySet) target;
        } else {
            throw new ODataApplicationException("Singletons not supported", HttpStatusCode.NOT_IMPLEMENTED.getStatusCode(),
                                                 Locale.ROOT);
        }
    }

    return entitySet;
}

7.2. Find the last navigation segment

public static UriResourceNavigation getLastNavigation(final UriInfoResource uriInfo) {

    final List<UriResource> resourcePaths = uriInfo.getUriResourceParts();
    int navigationCount = 1;
    while (navigationCount < resourcePaths.size()
        && resourcePaths.get(navigationCount) instanceof UriResourceNavigation) {
        navigationCount++;
    }

    return (UriResourceNavigation) resourcePaths.get(--navigationCount);
}

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