> ## Documentation Index
> Fetch the complete documentation index at: https://forest-chore-open-api.mintlify.site/llms.txt
> Use this file to discover all available pages before exploring further.
# Relationships
## 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](https://guides.rubyonrails.org/association_basics.html) to create new ones.
If you installed Forest directly on a database, then most relationships should have been [automatically generated](/legacy/javascript-agents/reference-guide/models/relationships/overview#lumber-relationship-generation-rules). 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](https://sequelize.org/master/manual/assocs.html) on adding relationships in your models (SQL)
* [Mongoose's documentation](https://mongoosejs.com/docs/guide.html) on adding relationships in your models (Mongodb)
### Adding a `hasMany` relationship
In our [Live demo](https://app.forestadmin.com/Live%20Demo/Production/Operations/data/806052/index), a **customer** can have multiple **orders**. In that case, we have to use a `hasMany` relationship.
```javascript theme={null}
module.exports = (sequelize, DataTypes) => {
const Customer = sequelize.define('customers',
...
);
Customer.associate = (models) => {
Customer.hasMany(models.orders);
};
return Customer;
};
```
```javascript theme={null}
module.exports = (mongoose, Mongoose) => {
const schema = Mongoose.Schema({
...
'orders': [{ type: Mongoose.Schema.Types.ObjectId, ref: 'orders' }],
...
}, {
timestamps: true,
});
return mongoose.model('customers', schema, 'customers');
};
```
Note that for orders to be displayed within the related data section of your customer, they have to be populated in your database. For instance:
Once you've added your relationship(s) in your model(s), they will only be taken into account **after you restart your server**.
### 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 **customer**`hasOne` **address"**.
```javascript theme={null}
module.exports = (sequelize, DataTypes) => {
const Customer = sequelize.define('customers',
...
);
Customer.associate = (models) => {
Customer.hasOne(models.addresses);
};
return Customer;
};
```
```javascript theme={null}
module.exports = (mongoose, Mongoose) => {
const schema = Mongoose.Schema({
...
'address': { type: Mongoose.Schema.Types.ObjectId, ref: 'addresses' },
...
}, {
timestamps: true,
});
return mongoose.model('customers', schema, 'customers');
};
```
Don't forget to **restart your server** for your newly added relationships to be taken into account.
### 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 **address**`belongsTo` a **customer**.
```javascript theme={null}
module.exports = (sequelize, DataTypes) => {
const Address = sequelize.define('addresses',
...
);
Address.associate = (models) => {
Address.belongsTo(models.customers);
};
return Address;
};
```
```javascript theme={null}
module.exports = (mongoose, Mongoose) => {
const schema = Mongoose.Schema({
...
'customer_id': { type: Mongoose.Schema.Types.ObjectId, ref: 'customers' },
...
}, {
timestamps: true,
});
return mongoose.model('addresses', schema, 'addresses');
};
```
This will work if your foreign keys are correctly named:\
For a collection `collectionName`, the foreign key should be `collection_name_id`.\
\
If this is not the case, check out the [section below](/legacy/javascript-agents/reference-guide/models/relationships/overview#declaring-a-foreign-key-sql-only).
Don't forget to **restart your server** for your newly added relationships to be taken into account.
#### 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:
```javascript theme={null}
...
Address.associate = (models) => {
Address.belongsTo(models.customers, {
foreignKey: 'fk_companyname'
targetKey: 'name'
});
};
...
```
This is explained in [Sequelize's documentation](https://sequelize.org/master/manual/associations.html#target-keys).
### Adding a `belongsToMany` relationship (SQL only)
`belongsToMany` association is often used to set up a many-to-many relationship with another model. For this example, we will consider the models `Projects` and `Users`. A user can be part of many projects, and one project has many users. The junction table that will keep track of the associations will be called `userProjects`, which will contain the foreign keys projectId and userId.
```javascript theme={null}
...
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',
});
};
...
```
```javascript theme={null}
...
Users.associate = (models) => {
Users.belongsToMany(models.projects, {
through: 'userProjects',
foreignKey: 'userId',
otherKey: 'projectId',
});
};
...
```
```javascript theme={null}
...
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:
**BelongsTo**
Detecting `belongsTo` is straight forward, we check if the referenced table of the foreign key is unique (unique constraint or primary key), then a `belongsTo` association can be set between the two tables.
**HasMany**
If the foreign key doesn't have a uniqueness constraint, then we can define a `hasMany` association.
**HasOne**
If the foreign key also have a unique constraint or is used as the primary key of its table, then we can define a `hasOne` association.
**BelongsToMany**
We detect Many-to-Many relationships when we detect a simple **junction table**. We are able to detect a junction table when it contains 2 foreign keys. It can optionally contain additional fields like a primary key and technical timestamps.
**BelongsTo**
When a document contains an ObjectID referring to another document, we create a `belongsTo` relationship to the corresponding collection.
**HasMany**
When a document contains an array of ObjectIDs referring to other documents, we create a `hasMany` relationship to the corresponding collection.
**HasOne**
Not automatically generated.
**BelongsToMany**
Not automatically generated.