Defining mixins

Overview

Use mixins to apply common logic to a set of models. For example, a timestamp mixin could inject “created” and “modified” properties to model definitions.

You can apply mixins to any model, including built-in models.

Built-in model mixins

Basic model

By default, the basic LoopBack Model object has properties and methods “mixed in” from:

• Inclusion object - Enables you to load relations of several objects and optimize numbers of requests.
• Validateable object - provides validation methods. See Validating model data.

When you define relations between models, the RelationMixin object object also gets mixed in to the model object.

Connected model

In addition to the methods of the Basic model object, the following are mixed in when a model is connected to a data source:

• RelationMixin class
• PersistedModel class

Create a mixin script

Mixin scripts are JavaScript files in one of the following folders, depending on the scope of the mixin:

• common/mixins/<mixin-Name>.js, for example common/mixins/timeStamp.js.
• server/mixins/<mixin-Name>.js, for example server/mixins/timeStamp.js.

If the mixin applies to both client and server models, put it in the common/mixins directory. If it applies only to server models, put it in the server/mixins directory.

You can use mixins to perform different common actions on models such as observing changes using operation hooks and adding model attributes.

For example:

module.exports = function(Model, options) {
  // Model is the model class
  // options is an object containing the config properties from model definition
  Model.defineProperty('created', {type: Date, default: '$now'});
  Model.defineProperty('modified', {type: Date, default: '$now'});
}

Above Timestamp mixin adds two properties: 'created' and 'modified' of type: date, with default values set to the current date, to Model.json.

Where as, in the example below, the mixin observes the change using before save operation hook and manipulates input (see complete example: loopback-example-mixins):

module.exports = function(Model, options) {
  'use strict';
  Model.observe('before save', function event(ctx, next) { //Observe any insert/update event on Model
    if (ctx.instance) {
      ctx.instance.squirrel = true;
    } else {
      ctx.data.squirrel = true;
    }
    next();
  });
};

Reference mixins in model-config.js

The configuration file server/model-config.json specifies the list of directories to be searched for mixin scripts. The default LoopBack configuration is:

{
  "_meta": {
    "sources": [
      "loopback/common/models",
      "loopback/server/models",
      "../common/models",
      "./models"
    ],
    "mixins": [
      "loopback/common/mixins",
      "loopback/server/mixins",
      "../common/mixins",
      "./mixins"
    ]
  },
  ...
}

Enable a model with mixins

To apply a mixin to a model, add “mixins” to the model definition JSON file. The value of mixins is an object keyed by normalized mixin names. The value for each mixin is passed into the script as the options argument and these options are implemented by mixins.

{
  "name": "note",
  "base": "PersistedModel",
  ...
  "mixins": {
     "Timestamp": {
       "myOption": 1,
       "anotherOpt": 2
     }
   },
  "properties": {
    ...
  },
  ...
}

In the example above, common/mixins/timestamp.js will be invoked with (note, {"myOption": 1, "anotherOpt":2})