Associations

Associations are relationships beteween two entities, termed “navigation properties” in Entity Framework documentation.

The designer supports creation of both unidirectional and bidirectional associations. In unidirectional associations, the source entity contains a property of the destination type, but not the other way around. In bidirectional associations, each class contains a property pointing to the other entity.

As an example, consider the following:

This tells us that Entity1 has a collection of Entity2 objects named TheEntity2s. The type of collection will typically be set as the default value set in the design surface, but can be overridden (see the Properties tables, below).

In short, this means that Entity1 would contain

public virtual ICollection<Entity2> TheEntity2s { get; set; }

but Entity2 doesn’t have a matching property.

In contrast, a bidirectional association like

would have, in the Entity2 class,

public virtual Entity1 TheEntity1 { get; set; }  // Required

as well as the collection of Entity2 objects in the Entity1 class. (Phew! Extensive overuse of the word Entity !)

Cardinalities

Associations have cardinalities (the number of elements in a set) at each end. Supported cardinalities are:

While cardinalities could technically include One or More (1..*), Entity Framework currently doesn’t directly support that. That doesn’t mean that it can’t be done, just that it has to be done with custom code (and, in fact, is on the to-do list of things to support with custom code generation in the designer).

Regardless of whether the association is unidirectional or bidirectional, it’s important to correctly specify the cardinality at each end. It has an effect on the kind of database structure Entity Framework creates, as well as being critical to properly modelling the problem space.

It can’t be emphasized enough - getting an association’s cardinality correct is extremely important. Lots of application code will have to change if you wind up changing the cardinalities after the entities are in use.

Unidirectional Association Properties

Unidirectional associations have the following properties:

PropertyDescription
Code Generation
Collection Class String. The concrete class used to implement Zero or More cardinality ends. Defaults to class set at the designer level, but can be overridden here. Must implement ICollection<T>.
Persistent Boolean. If false, the association will not be persisted in the database, requiring custom code to implement.
Documentation
Comment Summary String. XML comment <Summary> section
End1
End1 Multiplicity String. Cardinality of this end
End1 On Delete String. Only appears if End1's role is Principal. Describes how to handle dependent objects (the ones on the other end of the association) if the principal object (the one on on this end) is deleted.
Options are
  • None: don't cascade delete,
  • Cascade: always cascade delete, and
  • Default: cascade delete per Entity Framework rules depending on cardinality.
See the writeups for EF6 and EFCore for more information
End1 Role String. Whether this end is the Principal or Dependent end of the association. For more information, see Entity Framework Relationships and Navigation Properties
End2
End1 Custom Attributes String. Attributes generated in the code for this element - anything here will be generated verbatim into the code in the class definition.
End1 Display Text String. Will cause the generation of a [Display(Name="<text>")] attribute tag in the code.
End1 Navigation Property String. Name of the property that will be generated in the class on the *other* side of the association.
End2 Comment Detail String. XML comment <Remarks> section for this end
End2 Comment Summary String. XML comment <Summary> section for this end
End2 Multiplicity String. Cardinality of this end
End2 On Delete String. Only appears if End2's role is Principal. Describes how to handle dependent objects (the ones on the other end of the association) if the principal object (the one on on this end) is deleted.
Options are
  • None: don't cascade delete,
  • Cascade: always cascade delete, and
  • Default: cascade delete per Entity Framework rules depending on cardinality.
See the writeups for EF6 and EFCore for more information
End2 Role String. Whether this end is the Principal or Dependent end of the association. For more information, see Entity Framework Relationships and Navigation Properties

Bidirectional Association Properties

Bidirectional associations have the following properties:

PropertyDescription
Code Generation
Collection Class String. The concrete class used to implement Zero or More cardinality ends. Defaults to class set at the designer level, but can be overridden here. Must implement ICollection<T>.
Persistent Boolean. If false, the association will not be persisted in the database, requiring custom code to implement.
Documentation
Comment Summary String. XML comment <Summary> section
End1
End1 Comment Detail String. XML comment <Remarks> section for this end
End1 Comment Summary String. XML comment <Summary> section for this end
End1 Multiplicity String. Cardinality of this end
End1 On Delete String. Only appears if End1's role is Principal. Describes how to handle dependent objects (the ones on the other end of the association) if the principal object (the one on on this end) is deleted.
Options are
  • None: don't cascade delete,
  • Cascade: always cascade delete, and
  • Default: cascade delete per Entity Framework rules depending on cardinality.
See the writeups for EF6 and EFCore for more information
End1 Role String. Whether this end is the Principal or Dependent end of the association. For more information, see Entity Framework Relationships and Navigation Properties
End2 Custom Attributes String. Attributes generated in the code for this element - anything here will be generated verbatim into the code in the class definition.
End2 Display Text String. Will cause the generation of a [Display(Name="<text>")] attribute tag in the code.
End2 Navigation Property String. Name of the property that will be generated in the class on the *other* side of the association.
End2
End1 Custom Attributes String. Attributes generated in the code for this element - anything here will be generated verbatim into the code in the class definition.
End1 Display Text String. Will cause the generation of a [Display(Name="<text>")] attribute tag in the code.
End1 Navigation Property String. Name of the property that will be generated in the class on the *other* side of the association.
End2 Comment Detail String. XML comment <Remarks> section for this end
End2 Comment Summary String. XML comment <Summary> section for this end
End2 Multiplicity String. Cardinality of this end
End2 On Delete String. Only appears if End2's role is Principal. Describes how to handle dependent objects (the ones on the other end of the association) if the principal object (the one on on this end) is deleted.
Options are
  • None: don't cascade delete,
  • Cascade: always cascade delete, and
  • Default: cascade delete per Entity Framework rules depending on cardinality.
See the writeups for EF6 and EFCore for more information
End2 Role String. Whether this end is the Principal or Dependent end of the association. For more information, see Entity Framework Relationships and Navigation Properties

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Inheritance