Relationships - Forest Documentation

What is a relationship?

A relationship is a connection between two collections. Relationships are visible and actionable in Forest:

hasMany (1)
belongsTo or hasOne(2)

If you installed Forest within a Rails app, then all the relationships defined in your ActiveRecord models are supported out of the box. Check the official Rails documentation to create new ones. If you installed Forest directly on a database, then most relationships should have been automatically generated. However, depending on your database nature and structure, you may have to add some manually.

Adding relationships (databases only)

Depending on your database type, your models will have been generated in Sequelize (for SQL databases) or Mongoose (for Mongo databases). Below are some simple snippets showing you how to add relationships. However, should you want to dig deeper, please refer to the appropriate framework’s documentations:

Sequelize’s documentation on adding relationships in your models (SQL)
Mongoose’s documentation on adding relationships in your models (Mongodb)

Adding a `hasMany` relationship

In our Live demo, a customer can have multiple orders. In that case, we have to use a hasMany relationship.

Adding a `hasOne` relationship

In case of a one-to-one relationship between 2 collections, the opposite of a belongsTo relationship is a hasOne relationship. Taking the same example as before, the opposite of “an address belongsTo a customer” is simply “a customerhasOne address”.

Adding a `belongsTo` relationship

On our Live Demo example, the Address model has a foreignKey customer_id that points to the Customer. In other words, an addressbelongsTo a customer.

Declaring a foreign key (SQL only)

It’s possible that your tables are linked in an unusual way (using names instead of ids for instance).

In that case, adding the above code will not suffice to add the belongsTo relationship. Even though we recommend you modify your database structure to stay within foreign key conventions (pointing to an id), there is a way to specify how your tables are linked. If the field fk_customername of a table Address points to the field name of a table Customer, add the following:

...
UserProjects.associate = (models) => {
  UserProjects.belongsTo(models.projects, {
    foreignKey: {
      name: 'projectIdKey',
      field: 'projectId',
    },
    as: 'project',
  });
  UserProjects.belongsTo(models.users, {
    foreignKey: {
      name: 'userIdKey',
      field: 'userId',
    },
    as: 'user',
  });
};
...

...
Users.associate = (models) => {
  Users.belongsToMany(models.projects, {
    through: 'userProjects',
    foreignKey: 'userId',
    otherKey: 'projectId',
  });
};
...

...
Projects.associate = (models) => {
  Projects.belongsToMany(models.users, {
    through: 'userProjects',
    foreignKey: 'projectId',
    otherKey: 'userId',
  });
};
...

Relationship generation rules

Forest automatically generates most relationships, according to the below rules:

​What is a relationship?

​Adding relationships (databases only)

​Adding a hasMany relationship

​Adding a hasOne relationship

​Adding a belongsTo relationship

​Declaring a foreign key (SQL only)

​Relationship generation rules