How to: Create an association between LINQ to SQL classes (O/R Designer)
Applies to: 
Visual Studio 
Visual Studio for Mac
Note
This article applies to Visual Studio 2017. If you're looking for the latest Visual Studio documentation, see Visual Studio documentation. We recommend upgrading to the latest version of Visual Studio. Download it here
Associations between entity classes in LINQ to SQL are analogous to relationships between tables in a database. You can create associations between entity classes by using the Association Editor dialog box.
You must select a parent class and child class when you use the Association Editor dialog box to create an association. The parent class is the entity class that contains the primary key; the child class is the entity class that contains the foreign-key. For example, if entity classes were created that map to the Northwind Customers and Orders tables, the Customer class would be the parent class and the Order class would be the child class.
Note
When you drag tables from Server Explorer or Database Explorer onto the Object Relational Designer (O/R Designer), associations are automatically created based on the existing foreign-key relationships in the database.
Association properties
After you create an association, when you select the association in the O/R Designer, there are some configurable properties in the Properties window. (The association is the line between the related classes.) The following table provides descriptions for the properties of an association.
| Property | Description |
|---|---|
| Cardinality | Controls whether the association is one-to-many or one-to-one. |
| Child Property | Specifies whether to create a property on the parent that is a collection or reference to the child records on the foreign-key side of the association. For example, in the association between Customer and Order, if the Child Property is set to True, a property named Orders is created on the parent class. |
| Parent Property | The property on the child class that references the associated parent class. For example, in the association between Customer and Order, a property named Customer that references the associated customer for an order is created on the Order class. |
| Participating Properties | Displays the association properties and provides an ellipsis button (...) that re-opens the Association Editor dialog box. |
| Unique | Specifies whether the foreign target columns have a uniqueness constraint. |
To create an association between entity classes
Right-click the entity class that represents the parent class in the association, point to Add, and then click Association.
Verify that the correct Parent Class is selected in the Association Editor dialog box.
Select the Child Class in the combo box.
Select the Association Properties that relate the classes. Typically, this maps to the foreign-key relationship defined in the database. For example, in the
CustomersandOrdersassociation, the Association Properties are theCustomerIDfor each class.Click OK to create the association.