Associations

Last modified: August 20, 2024

Introduction

An association describes a relation between entities. In the domain model, an association is represented by a line or arrow between two entities.

Associations between two external entities from the same data source are defined in the originating app and therefore automatically established when the entities are used in a model. For further details, see the Associations section of External Entities.

Ownership

The value of an association should be viewed and edited from the object of the entity that is the owner of the association. Ownership in an association is signified by an arrow (note that the arrow does not signify direction). Either one entity or both entities can be the owner of the association. If one entity is the owner, there is an arrow that points from the owner to the other entity. If both entities are owners, there is a line between the two entities but no arrow. This is the only way the arrow can be controlled.

It is important to understand why ownership exists. Ownership is implemented in Mendix so that you can change relationships dynamically rather than being stuck with your first design. For example, if you design something as a one-to-many association and then need it to be a many-to-many association with default ownership, you do not need to rebuild your database, because Mendix handles it for you.

Multiplicity

The multiplicity (or number of referred objects) of an association is indicated by the number one (1) or a star (*) at either side of the association.

In the example below, the arrow indicates that Order is the owner of the association, and the 1 and * indicate that one customer is associated with many orders:

An association between a persistable entity and a non-persistable entity must start in the non-persistable entity and have the owner Default. For more information on persistable and non-persistable entities, see Persistability.

Creating Associations

The quickest way to create an association is to draw the association between two entities in a domain model. By default this will create a one-to-many association starting at the owner/many side of the association and ending at the one side of the association. The association will be named by joining the names of the two entities with an underscore. You can then edit the association as discussed in the next section.

You can also create associations between entities in different modules of your app. In this case it is not possible to draw the association. You can create an association to an entity in another module’s domain model by creating a new association in the Association tab of the entity which owns the association. You can then choose any entity within your app as the target of the association. For more information see Association Tab Properties.

You can create and edit associations only between an external entity and a local entity. However, an external entity cannot be the owner for an association with a local entity.

If you need to connect two external entities, consider adding a local entity and connect this local entity with both external entities. The local entity must be the owner of both associations, in this case.

Editing Associations

There are two ways of editing an association.

Edit the Association Directly

You can edit the association itself. In this case you will define the association using multiplicity and navigability.

For more information see Association Properties.

Edit from Associations in the Entity

You can edit the associations as members of the entity. In this case you will define the association using type and owner.

For more information see Association Tab Properties.

Move the Association Arrow

You can move the association arrow between two entities without deleting it. Click within the black dot on either side of the association arrow and drag it to the new desired location.

Your mouse pointer should not activate a white dot. This indicates the creation of a new association.

Reverse the Association Direction

In Mendix 10.3.0 and above, you can reverse an association direction by right-clicking the association and selecting Reverse direction.

Reversing the direction of an association creates a new association structure and exchanges the ownership of the association. It will also change the name of the association to reflect this change. This means you need to change how you access the associated entities.

For example, say you have a (1-*) Order_Customer association, meaning one Customer object is associated with multiple Order objects so a customer can have multiple orders, but an order can only have one customer. When you reverse the association direction, it will be changed to (*-1) Customer_Order, indicating that one Order object is associated with multiple Customer objects so an order can have multiple customers, but a customer can only have one order.

In cases of bidirectional associations, such as many-to-many or one-to-one associations, where both entities refer to each other mutually, reversing the association has no significant impact on the association itself because both entities are owners.

In all cases, you should change how you access data, using the new association name to be able to access the associated entities.

Reversing the association will result in the loss of existing associations, but the objects themselves (the Order and Customer objects in the above example) will not be deleted.

Association Examples

One-to-Many Association

In this example, drawing an association from the Order entity to the Customer entity results in the following:

The type property has its default value Reference, and the owner (the Order entity) is Default. This is the same as having multiplicity set to One 'Customer' object is associated with multiple 'Order' objects so a customer can have multiple orders, but an order can only have one customer.

In XML, instances of these entities and their association look as follows (note that the association is only stored in the Order element):

<Order id="101">
	<number>1</number>
	<date>9/30/2008</date>
	<Order_Customer>id_201</Order_Customer>
</Order>

<Customer id="201">
	<fullname>Apple Inc.</fullname>
	<address>1 Infinite Loop</address>
	<telephonenumber>1-800-MY-APPLE</telephonenumber>
</Customer>

Many-to-Many Association with Default Ownership

A many-to-many association with default ownership is created by drawing an association and then setting the type property to Reference set and leaving the owner as Default.

In this example, a Customer can have multiple Groups, and a Group can have multiple Customers. This is the same as having multiplicity set to Multiple 'Group' objects are associated with multiple 'Customer' objects with Navigability set to 'Customer' objects refer to 'Group' objects:

In XML, instances of these entities and their associations look as follows (note that the association is only stored in the Customer element):

<Customer id="201">
	<fullname>Apple Inc.</name>
	<address>1 Infinite Loop</address>
	<telephonenumber>1-800-MY-APPLE</telephonenumber>
	<Customer_Group>id_301 id_302</Customer_Group>
</Customer>

<Group id="301">
	<name>Multinational corporations</name>
</Group>

<Group id="302">
	<name>Hardware suppliers</name>
</Group>

One-to-One Association

A one-to-one association is created by setting the owner property to Both (while leaving the type property at its default value Reference).

In this example, a Customer can have one Profile, and a Profile can have one Customer. This is the same as having multiplicity set to One 'Customer' object is associated with one 'Profile' object:

In XML, instances of these entities and their associations look as follows (note that the association is stored both in the Profile element and the Customer element):

<Profile id="401">
	<religion>Buddhism</religion>
	<job>Chief Executive Officer</job>
	<website>http://www.apple.com/ </website>
	<Customer_Profile>id_201</Customer_Profile>
</Profile>

<Customer id="201">
	<fullname>Steve Jobs</fullname>
	<address>1 Infinite Loop</address>
	<telephonenumber>1-800-MY-APPLE</telephonenumber>
	<Customer_Profile>id_401</Customer_Profile>
</Customer>

Many-to-Many Association with Dual Ownership

A many-to-many association where both entities are owners is created by setting the owner property to Both and the type property to Reference set.

In this example, an Accountant can have multiple Groups and a Group can have multiple Accountants. This is the same as having multiplicity set to Multiple 'Group' objects are associated with multiple 'Accountant' objects with Navigability set to 'Accountant' and 'Group' objects refer to each other:

In XML, instances of these entities and their association look as follows (note that the association is stored both in the Accountant element and the Group element):

<Accountant id="501">
	<idnumber>1</idnumber>
	<name>Earl Grey</name>
	<telephonenumber>1-800-EARL-GREY</telephonenumber>
	<Accountant_Group>id_301 id_302</Accountant_Group>
</Accountant>

<Accountant id="502">
	<idnumber>2</idnumber>
	<name>Scrooge McDuck</name>
	<telephonenumber>1-800-SCROOGE-MCDUCK</telephonenumber>
	<Accountant_Group>id_301 id_302</Accountant_Group>
</Accountant>

<Group id="301">
	<name>Multinational corporations</name>
	<Accountant_Group>id_501 id_502</Accountant_Group>
</Group>

<Group id="302">
	<name>Hardware suppliers</name>
	<Accountant_Group>id_501 id_502</Accountant_Group>
</Group>