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Create a One-to-One Relationship

One-to-one relationships allow you to extend an existing entity with additional information that is not yet in the database model.

A common scenario is when you want to extend an entity with more attributes and it’s not possible to add them to it. This happens when you are using a referenced entity from a different module, and adding more attributes don’t make sense in the referenced module. In this case, you create an entity in your module to store the additional information.

To create a one-to-one relationship to an existing entity:

  1. Create an Entity;
  2. Change the Id attribute to be the identifier of the entity you want to extend;
  3. Add the attributes.


The GoOutWeb application allows to rate and review places like restaurants and hotels. The application saves some end-users information like the email, the phone number or twitter account.

Since we are using the User system entity to store the end-users it is not possible to add attributes to it. We have to create an additional entity to store more information about the user. To do so, follow these steps:

  1. In the Data tab, open the GoOutWebDataModel entity diagram;
  2. If not present yet, drag the User system entity to the diagram from the Data tab;
  3. Right-click on the diagram canvas and select Add Entity;
  4. Name it Profile;
  5. Rename the Id attribute to UserId.
    1. OutSystems sets the data type to User Identifier based on the name given;
    2. Since this data type is the identifier of the User entity, OutSystems creates a connection between the created entity Profile and the User entities.
  6. Add the attributes to the Profile entity:
    1. Twitter, Text type;
    2. Facebook, Text type;
    3. Photo, Binary Data type.

As a result, you have the Profile entity extending the User entity. Every time a Profile record is created its identifier should have the value of the User record identifier it belongs to.

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