Querying related models make unnecessary requests

Steps to reproduce

Create a model called Book with the properties id,author,title
Add a hasMany relation between the default User model and the Book model using a through Model called UserToBook which has the following properties: userId,bookId

The relation is defined in the User model like so.

    "books": {
      "type": "hasMany",
      "model": "Book",
      "foreignKey": "userId",
      "through": "UserToBook",
      "keyThrough": "bookId"
    },

Current Behavior

Querying the user for his books using a where filter performs 2 requests - one to the through model and one to the actual related model. The query to the through model is adding an include to the related model, which causes a fetch of a lot of data from the related model.
The reason this data is needed, is to extract the related model ids, so we can use it to filter out when performing the actual call the related model.

Expected Behavior

There is no need at all to fetch the related model data in the first call - we can simply get the ids from the through model, and use them

Link to reproduction sandbox

Working on it

Additional information

Since I'm using (sadly) loopback 2.x I have added the fix to my own fork - https://github.com/regevbr/loopback-datasource-juggler/blob/2.x/lib/scope.js
You can understand and base the fix based on the changes there

https://github.com/regevbr/loopback-datasource-juggler/blob/2cf8269e41a153d01e4d505af586bad9bb9110dd/lib/scope.js#L100-L138

Comienza leyendo lib/scope.js, especialmente el comportamiento mostrado en el commit enlazado y en el 2.x fork reportado. Reproduce la hasMany-through query con un where filter y verifica que la through-model request devuelva solo los IDs relacionados sin obtener los datos completos del related-model, mientras que la consulta final del related-model siga aplicando el filtro.