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Reflexive associations

Overview

In most cases, an association will be between attributes of two different models -- for example, a relationship between a User model and a Pet model. However, it is also possible to have a relationship between two attributes in the same model. This is called a reflexive association.

Consider the following User model:

// myApp/api/models/User.js
module.exports = {
  attributes: {
    firstName: {
      type: 'string'
    },
    lastName: {
      type: 'string'
    },

    // Add a singular reflexive association
    bestFriend: {
      model: 'user',
    },

    // Add one side of a plural reflexive association
    parents: {
      collection: 'user',
      via: 'children'
    },

    // Add the other side of a plural reflexive association
    children: {
      collection: 'user',
      via: 'parents'
    },

    // Add another plural reflexive association, this one via-less
    bookmarkedUsers: {
      collection: 'user'
    }

  }
};

The reflexive associations in the example User model above operate just like any other associations. The singular bestFriend attribute can be set to the primary key of another user (or for the narcissistic, to the same user!). The parents and children attributes can be modified using .addToCollection(), .removeFromCollection() and .replaceCollection(). Note that as with all plural associations, adding to one side will allow the relationship to be accessed by either side, so running:

// Add User #12 as a parent of User #23
await User.addToCollection(23, 'parents', 12);
// Find User #12 and populate its children
var userTwelve = await User.findOne(12).populate('children');

would return something like:

{
  id: 12,
  firstName: 'John',
  lastName: 'Doe',
  bestFriend: null,
  children: [
    {
      id: 23,
      firstName: 'Jane',
      lastName: 'Doe',
      bestFriend: null
    }
  ]
}

Notes

As with all “via-less” plural associations, reflexive via-less associations are only accessible from the side on which they are declared. In the above User model, you can do User.findOne(55).populate('bookmarkedUsers') to find all of the users that User #55 bookmarked, but there’s no way to get a list of all of the users that have bookmarked User #55. To do so would require an additional attribute (e.g. bookmarkedBy) that would be joined to bookmarkedUsers using the via property.

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