Has One Through

The "has-one-through" relationship defines a one-to-one relationship with another model. However, this relationship indicates that the declaring model can be matched with one instance of another model by proceeding through a third model.

For example, in a vehicle repair shop application, each Mechanic model may be associated with one Car model, and each Car model may be associated with one Owner model. While the mechanic and the owner have no direct relationship within the database, the mechanic can access the owner through the Car model. Let's look at the tables necessary to define this relationship:

mechanics
    id - integer
    name - string

cars
    id - integer
    model - string
    mechanic_id - integer

owners
    id - integer
    name - string
    car_id - integer

Now that we have examined the table structure for the relationship, let's define the relationship on the Mechanic model:

import {FedacoRelationType} from "./fedaco-types";

class Mechanic extends Model {
  /**
   * Get the car's owner.
   */
  @HasOneThroughColumn({
    related: forwardRef(() => Owner),
    through: forwardRef(() => Car)
  })
  public carOwner: FedacoRelationType<Owner>;
}

The related argument passed to the HasOneThroughColumn annotation is the name of the final model we wish to access, while the through argument is the name of the intermediate model.

Key Conventions

Typical Eloquent foreign key conventions will be used when performing the relationship's queries. If you would like to customize the keys of the relationship, you may pass them as the third and fourth arguments to the hasOneThrough method. The third argument is the name of the foreign key on the intermediate model. The fourth argument is the name of the foreign key on the final model. The fifth argument is the local key, while the sixth argument is the local key of the intermediate model:

import {FedacoRelationType} from "./fedaco-types";

class Mechanic extends Model {
  /**
   * Get the car's owner.
   */
  @HasOneThroughColumn({
    related: forwardRef(() => Owner),
    through: forwardRef(() => Car),
    firstKey: 'mechanic_id', // Foreign key on the cars table...
    secondKey: 'car_id', // Foreign key on the owners table...
    localKey: 'id', // Local key on the mechanics table...
    secondLocalKey: 'id' // Local key on the cars table...
  })
  public carOwner: FedacoRelationType<any>
}