4.5 Abstract entity types

Since Web API OData V5.5-beta, it is allowed to:

define abstract entity types without keys.
define abstract type (entity & complex) without any properties.
define derived entity types with their own keys.

Let’s see some examples:

Entity type example:

The CLR model is shown as below:

public abstract class Animal
{
}

public class Dog : Animal
{
  public int DogId { get; set; }
}

public class Pig : Animal
{
  public int PigId { get; set; }
}

We can use the following codes to build Edm Model:

  var builder = new ODataConventionModelBuilder();
  builder.EntityType<Animal>();
  builder.EntitySet<Dog>("Dogs");
  builder.EntitySet<Pig>("Pigs");
  IEdmModel model = builder.GetEdmModel()

Then, we can get the metadata document for Animal as:

<EntityType Name="Animal" Abstract="true" />
<EntityType Name="Dog" BaseType="NS.Animal">
    <Key>
        <PropertyRef Name="DogId" />
    </Key>
    <Property Name="DogId" Type="Edm.Int32" Nullable="false" />
</EntityType>
<EntityType Name="Pig" BaseType="NS.Animal">
    <Key>
        <PropertyRef Name="PigId" />
    </Key>
    <Property Name="PigId" Type="Edm.Int32" Nullable="false" />
</EntityType>

Note:

Animal is an abstract entity type without any keys and any properties
Dog & Pig are two sub entity types derived from Animal with own keys.

However, it’s obvious that abstract entity type without keys can’t be used to define any navigation sources (entity set or singleton). So, if you try to:

builder.EntitySet<Animal>("Animals");

you will get the following exception:

System.InvalidOperationException: The entity set or singlet on 'Animals' is based on type 'NS.Animal' that has no keys defined.

Complex type example

Let’s see a complex example. The CLR model is shown as below:

public abstract class Graph
{ }

public class Point : Graph
{
  public int X { get; set; }
  public int Y { get; set; }
}

public class Line : Graph
{
  public IList<Point> Vertexes { get; set; }
}

We can use the following codes to build Edm Model:

  var builder = new ODataConventionModelBuilder();
  builder.ComplexType<Graph>();
  IEdmModel model = builder.GetEdmModel()

Then, we can get the metadata document for Graph as:

<ComplexType Name="Graph" Abstract="true" />
<ComplexType Name="Point" BaseType="NS.Graph">
    <Property Name="X" Type="Edm.Int32" Nullable="false" />
    <Property Name="Y" Type="Edm.Int32" Nullable="false" />
</ComplexType>
<ComplexType Name="Line" BaseType="NS.Graph">
    <Property Name="Vertexes" Type="Collection(NS.Point)" />
</ComplexType>

Where, Graph is an abstract complex type without any properties.

Thanks.