sequelize/lib/associations/belongs-to-many.js

Multi dialect ORM for Node.JS/io.js

'use strict';

var Utils = require('./../utils')
  , Helpers = require('./helpers')
  , _ = require('lodash')
  , Association = require('./base')
  , BelongsTo = require('./belongs-to')
  , HasMany = require('./has-many')
  , HasOne = require('./has-one')
  , CounterCache = require('../plugins/counter-cache')
  , util = require('util');

/**
 * Many-to-many association with a join table.
 *
 * When the join table has additional attributes, these can be passed in the options object:
 *
 * ```js
 * UserProject = sequelize.define('user_project', {
 *   role: Sequelize.STRING
 * });
 * User.belongsToMany(Project, { through: UserProject });
 * Project.belongsToMany(User, { through: UserProject });
 * // through is required!
 *
 * user.addProject(project, { role: 'manager', transaction: t });
 * ```
 *
 * All methods allow you to pass either a persisted instance, its primary key, or a mixture:
 *
 * ```js
 * Project.create({ id: 11 }).then(function (project) {
 *   user.addProjects([project, 12]);
 * });
 * ```
 *
 * In the API reference below, replace `Assocation(s)` with the actual name of your association, e.g. for `User.belongsToMany(Project)` the getter will be `user.getProjects()`.
 *
 * @mixin BelongsToMany
 */
var BelongsToMany = function(source, target, options) {
  Association.call(this);

  options = options || {};

  if (options.through === undefined || options.through === true || options.through === null) {
    throw new Error('belongsToMany must be given a through option, either a string or a model');
  }

  if (!options.through.model) {
    options.through = {
      model: options.through
    };
  }

  this.associationType = 'BelongsToMany';
  this.source = source;
  this.target = target;
  this.targetAssociation = null;
  this.options = options;
  this.sequelize = source.modelManager.sequelize;
  this.through = _.assign({}, options.through);
  this.scope = options.scope;
  this.isMultiAssociation = true;
  this.isSelfAssociation = this.source === this.target;
  this.doubleLinked = false;
  this.as = this.options.as;

  if (!this.as && this.isSelfAssociation) {
    throw new Error('\'as\' must be defined for many-to-many self-associations');
  }

  if (this.as) {
    this.isAliased = true;

    if (Utils._.isPlainObject(this.as)) {
      this.options.name = this.as;
      this.as = this.as.plural;
    } else {
      this.options.name = {
        plural: this.as,
        singular: Utils.singularize(this.as)
      };
    }
  } else {
    this.as = this.target.options.name.plural;
    this.options.name = this.target.options.name;
  }

  this.combinedTableName = Utils.combineTableNames(
    this.source.tableName,
    this.isSelfAssociation ? (this.as || this.target.tableName) : this.target.tableName
  );

  /*
   * If self association, this is the target association - Unless we find a pairing association
   */
  if (this.isSelfAssociation) {
    this.targetAssociation = this;
  }

  /*
   * Default/generated foreign/other keys
   */
  if (_.isObject(this.options.foreignKey)) {
    this.foreignKeyAttribute = this.options.foreignKey;
    this.foreignKey = this.foreignKeyAttribute.name || this.foreignKeyAttribute.fieldName;
  } else {
    if (!this.options.foreignKey) {
      this.foreignKeyDefault = true;
    }

    this.foreignKeyAttribute = {};
    this.foreignKey = this.options.foreignKey || Utils.camelizeIf(
      [
        Utils.underscoredIf(this.source.options.name.singular, this.source.options.underscored),
        this.source.primaryKeyAttribute
      ].join('_'),
      !this.source.options.underscored
    );
  }

  if (_.isObject(this.options.otherKey)) {
    this.otherKeyAttribute = this.options.otherKey;
    this.otherKey = this.otherKeyAttribute.name || this.otherKeyAttribute.fieldName;
  } else {
    if (!this.options.otherKey) {
      this.otherKeyDefault = true;
    }

    this.otherKeyAttribute = {};
    this.otherKey = this.options.otherKey || Utils.camelizeIf(
      [
        Utils.underscoredIf(
          this.isSelfAssociation ?
            Utils.singularize(this.as) :
            this.target.options.name.singular,
          this.target.options.underscored
        ),
        this.target.primaryKeyAttribute
      ].join('_'),
      !this.target.options.underscored
    );
  }

  /*
   * Find paired association (if exists)
   */
  _.each(this.target.associations, function(association) {
    if (association.associationType !== 'BelongsToMany') return;
    if (association.target !== this.source) return;

    if (this.options.through.model === association.options.through.model) {
      this.paired = association;
      association.paired = this;
    }
  }.bind(this));

  if (typeof this.through.model === 'string') {
    if (!this.sequelize.isDefined(this.through.model)) {
      this.through.model = this.sequelize.define(this.through.model, {}, _.extend(this.options, {
        tableName: this.through.model,
        indexes: [], //we don't want indexes here (as referenced in #2416)
        paranoid: false,  // A paranoid join table does not make sense
        validate: {} // Don't propagate model-level validations
      }));
    } else {
      this.through.model = this.sequelize.model(this.through.model);
    }
  }

  this.options = _.assign(this.options, _.pick(this.through.model.options, [
    'timestamps', 'createdAt', 'updatedAt', 'deletedAt', 'paranoid'
  ]));

  if (this.paired) {
    if (this.otherKeyDefault) {
      this.otherKey = this.paired.foreignKey;
    }
    if (this.paired.otherKeyDefault) {
      // If paired otherKey was inferred we should make sure to clean it up before adding a new one that matches the foreignKey
      if (this.paired.otherKey !== this.foreignKey) {
        delete this.through.model.rawAttributes[this.paired.otherKey];
      }
      this.paired.otherKey = this.foreignKey;
      this.paired.foreignIdentifier = this.foreignKey;
      delete this.paired.foreignIdentifierField;
    }
  }

  if (this.through) {
    this.throughModel = this.through.model;
  }

  this.options.tableName = this.combinedName = (this.through.model === Object(this.through.model) ? this.through.model.tableName : this.through.model);

  this.associationAccessor = this.as;

  // Get singular and plural names, trying to uppercase the first letter, unless the model forbids it
  var plural = Utils.uppercaseFirst(this.options.name.plural)
    , singular = Utils.uppercaseFirst(this.options.name.singular);

  this.accessors = {
    /**
     * Get everything currently associated with this, using an optional where clause.
     *
     * @param {Object} [options]
     * @param {Object} [options.where] An optional where clause to limit the associated models
     * @param {String|Boolean} [options.scope] Apply a scope on the related model, or remove its default scope by passing false
     * @param {String} [options.schema] Apply a schema on the related model
     * @return {Promise<Array<Instance>>}
     * @method getAssociations
     */
    get: 'get' + plural,
    /**
     * Set the associated models by passing an array of instances or their primary keys. Everything that it not in the passed array will be un-associated.
     *
     * @param {Array<Instance|String|Number>} [newAssociations] An array of persisted instances or primary key of instances to associate with this. Pass `null` or `undefined` to remove all associations.
     * @param {Object} [options] Options passed to `through.findAll`, `bulkCreate`, `update` and `destroy`. Can also hold additional attributes for the join table
     * @param {Object} [options.validate] Run validation for the join model
     * @return {Promise}
     * @method setAssociations
     */
    set: 'set' + plural,
    /**
     * Associate several persisted instances with this.
     *
     * @param {Array<Instance|String|Number>} [newAssociations] An array of persisted instances or primary key of instances to associate with this.
     * @param {Object} [options] Options passed to `through.findAll`, `bulkCreate` and `update`. Can also hold additional attributes for the join table.
     * @param {Object} [options.validate] Run validation for the join model.
     * @return {Promise}
     * @method addAssociations
     */
    addMultiple: 'add' + plural,
    /**
     * Associate a persisted instance with this.
     *
     * @param {Instance|String|Number} [newAssociation] A persisted instance or primary key of instance to associate with this.
     * @param {Object} [options] Options passed to `through.findAll`, `bulkCreate` and `update`. Can also hold additional attributes for the join table.
     * @param {Object} [options.validate] Run validation for the join model.
     * @return {Promise}
     * @method addAssociation
     */
    add: 'add' + singular,
     /**
     * Create a new instance of the associated model and associate it with this.
     *
     * @param {Object} [values]
     * @param {Object} [options] Options passed to create and add. Can also hold additional attributes for the join table
     * @return {Promise}
     * @method createAssociation
     */
    create: 'create' + singular,
    /**
     * Un-associate the instance.
     *
     * @param {Instance|String|Number} [oldAssociated] Can be an Instance or its primary key
     * @param {Object} [options] Options passed to `through.destroy`
     * @return {Promise}
     * @method removeAssociation
     */
    remove: 'remove' + singular,
    /**
     * Un-associate several instances.
     *
     * @param {Array<Instance|String|Number>} [oldAssociated] Can be an array of instances or their primary keys
     * @param {Object} [options] Options passed to `through.destroy`
     * @return {Promise}
     * @method removeAssociations
     */
    removeMultiple: 'remove' + plural,
    /**
     * Check if an instance is associated with this.
     *
     * @param {Instance|String|Number} [instance] Can be an Instance or its primary key
     * @param {Object} [options] Options passed to getAssociations
     * @return {Promise}
     * @method hasAssociation
     */
    hasSingle: 'has' + singular,
    /**
     * Check if all instances are associated with this.
     *
     * @param {Array<Instance|String|Number>} [instances] Can be an array of instances or their primary keys
     * @param {Object} [options] Options passed to getAssociations
     * @return {Promise}
     * @method hasAssociations
     */
    hasAll: 'has' + plural,
    /**
     * Count everything currently associated with this, using an optional where clause.
     *
     * @param {Object} [options]
     * @param {Object} [options.where] An optional where clause to limit the associated models
     * @param {String|Boolean} [options.scope] Apply a scope on the related model, or remove its default scope by passing false
     * @return {Promise<Int>}
     * @method countAssociations
     */
    count: 'count' + plural
  };

  if (this.options.counterCache) {
    new CounterCache(this, this.options.counterCache !== true ? this.options.counterCache : {});
  }
};

util.inherits(BelongsToMany, Association);

// the id is in the target table
// or in an extra table which connects two tables
BelongsToMany.prototype.injectAttributes = function() {
  var self = this;

  this.identifier = this.foreignKey;
  this.foreignIdentifier = this.otherKey;

  // remove any PKs previously defined by sequelize
  // but ignore any keys that are part of this association (#5865)
  _.each(this.through.model.rawAttributes, function(attribute, attributeName) {
    if (attribute.primaryKey === true && attribute._autoGenerated === true) {
      if (attributeName === self.foreignKey || attributeName === self.otherKey) {
        // this key is still needed as it's part of the association
        // so just set primaryKey to false
        attribute.primaryKey = false;
      }
      else {
        delete self.through.model.rawAttributes[attributeName];
      }
      self.primaryKeyDeleted = true;
    }
  });

  var sourceKey = this.source.rawAttributes[this.source.primaryKeyAttribute]
    , sourceKeyType = sourceKey.type
    , sourceKeyField = sourceKey.field || this.source.primaryKeyAttribute
    , targetKey = this.target.rawAttributes[this.target.primaryKeyAttribute]
    , targetKeyType = targetKey.type
    , targetKeyField = targetKey.field || this.target.primaryKeyAttribute
    , sourceAttribute = _.defaults({}, this.foreignKeyAttribute, { type: sourceKeyType })
    , targetAttribute = _.defaults({}, this.otherKeyAttribute, { type: targetKeyType });

  if (this.primaryKeyDeleted === true) {
    targetAttribute.primaryKey = sourceAttribute.primaryKey = true;
  } else if (this.through.unique !== false) {
    var uniqueKey = [this.through.model.tableName, this.foreignKey, this.otherKey, 'unique'].join('_');
    targetAttribute.unique = sourceAttribute.unique = uniqueKey;
  }

  if (!this.through.model.rawAttributes[this.foreignKey]) {
    this.through.model.rawAttributes[this.foreignKey] = {
      _autoGenerated: true
    };
  }

  if (!this.through.model.rawAttributes[this.otherKey]) {
    this.through.model.rawAttributes[this.otherKey] = {
      _autoGenerated: true
    };
  }

  if (this.options.constraints !== false) {
    sourceAttribute.references = {
      model: this.source.getTableName(),
      key:   sourceKeyField
    };
    // For the source attribute the passed option is the priority
    sourceAttribute.onDelete = this.options.onDelete || this.through.model.rawAttributes[this.foreignKey].onDelete;
    sourceAttribute.onUpdate = this.options.onUpdate || this.through.model.rawAttributes[this.foreignKey].onUpdate;

    if (!sourceAttribute.onDelete) sourceAttribute.onDelete = 'CASCADE';
    if (!sourceAttribute.onUpdate) sourceAttribute.onUpdate = 'CASCADE';

    targetAttribute.references = {
      model: this.target.getTableName(),
      key:   targetKeyField
    };
    // But the for target attribute the previously defined option is the priority (since it could've been set by another belongsToMany call)
    targetAttribute.onDelete = this.through.model.rawAttributes[this.otherKey].onDelete || this.options.onDelete;
    targetAttribute.onUpdate = this.through.model.rawAttributes[this.otherKey].onUpdate || this.options.onUpdate;

    if (!targetAttribute.onDelete) targetAttribute.onDelete = 'CASCADE';
    if (!targetAttribute.onUpdate) targetAttribute.onUpdate = 'CASCADE';
  }

  this.through.model.rawAttributes[this.foreignKey] = _.extend(this.through.model.rawAttributes[this.foreignKey], sourceAttribute);
  this.through.model.rawAttributes[this.otherKey] = _.extend(this.through.model.rawAttributes[this.otherKey], targetAttribute);

  this.identifierField = this.through.model.rawAttributes[this.foreignKey].field || this.foreignKey;
  this.foreignIdentifierField = this.through.model.rawAttributes[this.otherKey].field || this.otherKey;

  if (this.paired && !this.paired.foreignIdentifierField) {
    this.paired.foreignIdentifierField = this.through.model.rawAttributes[this.paired.otherKey].field || this.paired.otherKey;
  }

  this.through.model.refreshAttributes();

  this.toSource = new BelongsTo(this.through.model, this.source, {
    foreignKey: this.foreignKey
  });
  this.manyFromSource = new HasMany(this.source, this.through.model, {
    foreignKey: this.foreignKey
  });
  this.oneFromSource = new HasOne(this.source, this.through.model, {
    foreignKey: this.foreignKey,
    as: this.through.model.name
  });

  this.toTarget = new BelongsTo(this.through.model, this.target, {
    foreignKey: this.otherKey
  });
  this.manyFromTarget = new HasMany(this.target, this.through.model, {
    foreignKey: this.otherKey
  });
  this.oneFromTarget = new HasOne(this.target, this.through.model, {
    foreignKey: this.otherKey,
    as: this.through.model.name
  });

  if (this.paired && this.paired.otherKeyDefault) {
    this.paired.toTarget = new BelongsTo(this.paired.through.model, this.paired.target, {
      foreignKey: this.paired.otherKey
    });

    this.paired.oneFromTarget = new HasOne(this.paired.target, this.paired.through.model, {
      foreignKey: this.paired.otherKey,
      as: this.paired.through.model.name
    });
  }

  Helpers.checkNamingCollision(this);

  return this;
};

BelongsToMany.prototype.get = function(instance, options) {
  options = Utils.cloneDeep(options) || {};

  var association = this
    , through = association.through
    , scopeWhere
    , throughWhere;

  if (association.scope) {
    scopeWhere = _.clone(association.scope);
  }

  options.where = {
    $and: [
      scopeWhere,
      options.where
    ]
  };

  if (Object(through.model) === through.model) {
    throughWhere = {};
    throughWhere[association.foreignKey] = instance.get(association.source.primaryKeyAttribute);

    if (through.scope) {
      _.assign(throughWhere, through.scope);
    }

    //If a user pass a where on the options through options, make an "and" with the current throughWhere
    if (options.through && options.through.where) {
      throughWhere = {
        $and: [throughWhere, options.through.where]
      };
    }

    options.include = options.include || [];
    options.include.push({
      association: association.oneFromTarget,
      attributes: options.joinTableAttributes,
      required: true,
      where: throughWhere
    });
  }

  var model = association.target;
  if (options.hasOwnProperty('scope')) {
    if (!options.scope) {
      model = model.unscoped();
    } else {
      model = model.scope(options.scope);
    }
  }

  if (options.hasOwnProperty('schema')) {
    model = model.schema(options.schema, options.schemaDelimiter);
  }

  return model.findAll(options);
};

BelongsToMany.prototype.injectGetter = function(obj) {
  var association = this;

  obj[this.accessors.get] = function(options) {
    return association.get(this, options);
  };

  obj[this.accessors.count] = function(options) {
    var model = association.target
      , sequelize = model.sequelize;

    options = Utils.cloneDeep(options);
    options.attributes = [
      [sequelize.fn('COUNT', sequelize.col([association.target.name, model.primaryKeyAttribute].join('.'))), 'count']
    ];
    options.joinTableAttributes = [];
    options.raw = true;
    options.plain = true;

    return obj[association.accessors.get].call(this, options).then(function (result) {
      return parseInt(result.count, 10);
    });
  };

  obj[this.accessors.hasSingle] = obj[this.accessors.hasAll] = function(instances, options) {
    var where = {};

    if (!Array.isArray(instances)) {
      instances = [instances];
    }

    options = _.assign({
      raw: true
    }, options, {
      scope: false
    });

    where.$or = instances.map(function (instance) {
      if (instance instanceof association.target.Instance) {
        return instance.where();
      } else {
        var $where = {};
        $where[association.target.primaryKeyAttribute] = instance;
        return $where;
      }
    });

    options.where = {
      $and: [
        where,
        options.where
      ]
    };

    return this[association.accessors.get](options).then(function(associatedObjects) {
      return associatedObjects.length === instances.length;
    });
  };

  return this;
};

BelongsToMany.prototype.injectSetter = function(obj) {
  var association = this;

  obj[this.accessors.set] = function(newAssociatedObjects, options) {
    options = options || {};
    var instance = this
      , sourceKey = association.source.primaryKeyAttribute
      , targetKey = association.target.primaryKeyAttribute
      , identifier = association.identifier
      , foreignIdentifier = association.foreignIdentifier
      , where = {};

    if (newAssociatedObjects === null) {
      newAssociatedObjects = [];
    } else {
      newAssociatedObjects = association.toInstanceArray(newAssociatedObjects);
    }

    where[identifier] = this.get(sourceKey);
    _.assign(where, association.through.scope);
    
    return association.through.model.findAll(_.defaults({
      where: where,
      raw: true,
    }, options)).then(function (currentRows) {
      var obsoleteAssociations = []
        , defaultAttributes = options
        , promises = []
        , unassociatedObjects;

      // Don't try to insert the transaction as an attribute in the through table
      defaultAttributes = _.omit(defaultAttributes, ['transaction', 'hooks', 'individualHooks', 'ignoreDuplicates', 'validate', 'fields', 'logging']);

      unassociatedObjects = newAssociatedObjects.filter(function(obj) {
        return !_.find(currentRows, function(currentRow) {
          return currentRow[foreignIdentifier] === obj.get(targetKey);
        });
      });

      currentRows.forEach(function(currentRow) {
        var newObj = _.find(newAssociatedObjects, function(obj) {
          return currentRow[foreignIdentifier] === obj.get(targetKey);
        });

        if (!newObj) {
          obsoleteAssociations.push(currentRow);
        } else {
          var throughAttributes = newObj[association.through.model.name];
          // Quick-fix for subtle bug when using existing objects that might have the through model attached (not as an attribute object)
          if (throughAttributes instanceof association.through.model.Instance) {
            throughAttributes = {};
          }

          var where = {}
            , attributes = _.defaults({}, throughAttributes, defaultAttributes);

          where[identifier] = instance.get(sourceKey);
          where[foreignIdentifier] = newObj.get(targetKey);

          if (Object.keys(attributes).length) {
            promises.push(association.through.model.update(attributes, _.extend(options, {
              where: where
            })));
          }
        }
      });

      if (obsoleteAssociations.length > 0) {
        var where = {};
        where[identifier] = instance.get(sourceKey);
        where[foreignIdentifier] = obsoleteAssociations.map(function(obsoleteAssociation) {
          return obsoleteAssociation[foreignIdentifier];
        });

        promises.push(association.through.model.destroy(_.defaults({
          where: where
        }, options)));
      }

      if (unassociatedObjects.length > 0) {
        var bulk = unassociatedObjects.map(function(unassociatedObject) {
          var attributes = {};

          attributes[identifier] = instance.get(sourceKey);
          attributes[foreignIdentifier] = unassociatedObject.get(targetKey);

          attributes = _.defaults(attributes, unassociatedObject[association.through.model.name], defaultAttributes);

          _.assign(attributes, association.through.scope);

          return attributes;
        }.bind(this));

        promises.push(association.through.model.bulkCreate(bulk, _.assign({ validate: true }, options)));
      }

      return Utils.Promise.all(promises);
    });
  };

  obj[this.accessors.addMultiple] = obj[this.accessors.add] = function(newInstances, additionalAttributes) {
    // If newInstances is null or undefined, no-op
    if (!newInstances) return Utils.Promise.resolve();

    additionalAttributes = _.clone(additionalAttributes) || {};

    var instance = this
      , defaultAttributes = _.omit(additionalAttributes, ['transaction', 'hooks', 'individualHooks', 'ignoreDuplicates', 'validate', 'fields', 'logging'])
      , sourceKey = association.source.primaryKeyAttribute
      , targetKey = association.target.primaryKeyAttribute
      , identifier = association.identifier
      , foreignIdentifier = association.foreignIdentifier
      , options = additionalAttributes;

    newInstances = association.toInstanceArray(newInstances);

    var where = {};
    where[identifier] = instance.get(sourceKey);
    where[foreignIdentifier] = newInstances.map(function (newInstance) { return newInstance.get(targetKey); });

    _.assign(where, association.through.scope);

    return association.through.model.findAll(_.defaults({
      where: where,
      raw: true,
    }, options)).then(function (currentRows) {
      var promises = [];

      var unassociatedObjects = [], changedAssociations = [];
      newInstances.forEach(function(obj) {
        var existingAssociation = _.find(currentRows, function(current) {
          return current[foreignIdentifier] === obj.get(targetKey);
        });

        if (!existingAssociation) {
          unassociatedObjects.push(obj);
        } else {
          var throughAttributes = obj[association.through.model.name]
            , attributes = _.defaults({}, throughAttributes, defaultAttributes);

          if (_.some(Object.keys(attributes), function (attribute) {
            return attributes[attribute] !== existingAssociation[attribute];
          })) {
            changedAssociations.push(obj);
          }
        }
      });

      if (unassociatedObjects.length > 0) {
        var bulk = unassociatedObjects.map(function(unassociatedObject) {
          var throughAttributes = unassociatedObject[association.through.model.name]
            , attributes = _.defaults({}, throughAttributes, defaultAttributes);

          attributes[identifier] = instance.get(sourceKey);
          attributes[foreignIdentifier] = unassociatedObject.get(targetKey);

          _.assign(attributes, association.through.scope);

          return attributes;
        }.bind(this));

        promises.push(association.through.model.bulkCreate(bulk, _.assign({ validate: true }, options)));
      }

      changedAssociations.forEach(function(assoc) {
        var throughAttributes = assoc[association.through.model.name]
          , attributes = _.defaults({}, throughAttributes, defaultAttributes)
          , where = {};
        // Quick-fix for subtle bug when using existing objects that might have the through model attached (not as an attribute object)
        if (throughAttributes instanceof association.through.model.Instance) {
          throughAttributes = {};
        }

        where[identifier] = instance.get(sourceKey);
        where[foreignIdentifier] = assoc.get(targetKey);

        promises.push(association.through.model.update(attributes, _.extend(options, {
          where: where
        })));
      });

      return Utils.Promise.all(promises);
    });
  };

  obj[this.accessors.removeMultiple] = obj[this.accessors.remove] = function(oldAssociatedObjects, options) {
    options = options || {};

    oldAssociatedObjects = association.toInstanceArray(oldAssociatedObjects);

    var where = {};
    where[association.identifier] = this.get(association.source.primaryKeyAttribute);
    where[association.foreignIdentifier] = oldAssociatedObjects.map(function (newInstance) { return newInstance.get(association.target.primaryKeyAttribute); });

    return association.through.model.destroy(_.defaults({
      where: where
    }, options));
  };

  return this;
};

BelongsToMany.prototype.injectCreator = function(obj) {
  var association = this;

  obj[this.accessors.create] = function(values, options) {
    var instance = this;
    options = options || {};
    values = values || {};

    if (Array.isArray(options)) {
      options = {
        fields: options
      };
    }

    if (association.scope) {
      _.assign(values, association.scope);
      if (options.fields) {
        options.fields = options.fields.concat(Object.keys(association.scope));
      }
    }

    // Create the related model instance
    return association.target.create(values, options).then(function(newAssociatedObject) {
      return instance[association.accessors.add](newAssociatedObject, _.omit(options, ['fields'])).return(newAssociatedObject);
    });
  };

  return this;
};

module.exports = BelongsToMany;
module.exports.BelongsToMany = BelongsToMany;
module.exports.default = BelongsToMany;