sails postgresql many-to-many association not working

I'm trying to create a many to many association between two models, Operator and Group.

Two two models are:

-Operator.js

    var Operator = {
    connection:'postgresql',
    tableName: 'operator',
    schema:true,
    attributes: {

    firstName: {
        type: 'string',
        required: true,
        max: 64,
        columnName: 'first_name'
    },
    lastName: {
        type: 'string',
        required: true,
        max: 64,
        columnName: 'last_name'
    },
    birthDate: {
        type: 'date',
        columnName: 'birth_date'
    },
    sex: {
        type: 'string',
        enum: ['M', 'F', 'N.A.'],
        columnName: 'sex'
    },
    email: {
        type: 'email',
        required: true,
        columnName: 'email'
    },
    login: {
        type: 'string',
        required: true,
        max: 64
    },
    password: {
        type: 'string',
        required: true
    },

    createdAt: {
        columnName: 'created_at'
    },

    updatedAt: { 
        columnName: 'updated_at'
    },

    groups:{
        collection:'group',
        via:'operators'
    },

    // Override toJSON instance method
    // to remove password value
    toJSON: function() {
        var obj = this.toObject();
        delete obj.password;
        return obj;
    }


},

// Lifecycle Callbacks
beforeCreate: function(values, next) {
    bcrypt.hash(values.password, 10, function(err, hash) {
        if(err) return next(err);
        values.password = hash;
        next();
    });
}

};

module.exports = Operator;

-Group.js

    var bcrypt = require('bcrypt');

    var Group = {
    connection:'postgresql',
    tableName: 'group',
    schema:true,
    attributes: {
    name: {
        type: 'string',
        required: true,
        unique:true,
        columnName: 'name'
    },
    createdAt: {
        columnName: 'created_at'
    },
    updatedAt: {
        columnName: 'updated_at'
    },

    operators:
        {
        collection:'operator',
        via:'groups',
        dominant:true
    }
       }
 };
 module.exports = Group;

I created the join table group_operators__operator_groups that has the two columns group_operators and operator_groups.

On sails console I tried to make the association with this command:

     Operator.find(149).populate('groups').exec(function(e,r){
     console.log(e); 
     r[0].groups.add(19);
     console.log(r[0]); 
     r[0].save(function(err){
              console.log(err);})
     });

But in the result an error appeared:

   null
   { groups: [],
   sex: 'F',
   email: 'nhjfhry@t5rjyi.girtj',
   login: 'tryjrtyh',
   password: '$2a$10$3pOYcOpWWcU868LBB0Gki./n9nrooXyDqSNYz1NJCkvQ480KT5uxO',
   id: 149,
   firstName: 'fgterier',
   lastName: 'gdfgjdi',
   birthDate: Thu Sep 25 2014 00:00:00 GMT+0200 (CEST),
   createdAt: null,
   updatedAt: Thu Oct 02 2014 12:45:14 GMT+0200 (CEST) }
   [ { type: 'insert',
   collection: 'group_operators__operator_groups',
   criteria: { group_operators: 19, operator_groups: 149 },
   values: { group_operators: 19, operator_groups: 149 },
   err: [Error: Trying to '.add()' an instance which already exists!] } ]

   Possibly unhandled Error: [object Object]
   at Object.Promise$_rejecter [as reject]   (/usr/lib/node_modules/sails/node_modules/waterline/node_modules/bluebird/js/main/promise.js:601:58)
at /usr/lib/node_modules/sails/node_modules/waterline/lib/waterline/model/lib/defaultMethods/save.js:154:16
at /usr/lib/node_modules/sails/node_modules/waterline/node_modules/async/lib/async.js:454:17
at /usr/lib/node_modules/sails/node_modules/waterline/node_modules/async/lib/async.js:444:17
at Array.forEach (native)
at _each (/usr/lib/node_modules/sails/node_modules/waterline/node_modules/async/lib/async.js:46:24)
at Object.taskComplete (/usr/lib/node_modules/sails/node_modules/waterline/node_modules/async/lib/async.js:443:13)
at processImmediate [as _immediateCallback] (timers.js:345:15)

If I don't create the Join Table and I try to make the association,sails console returns this error:

   Error (E_UNKNOWN) :: Encountered an unexpected error
   error: relation "group_operators__operator_groups" does not exist
   at Connection.parseE (/home/valentina/workspace/xtens-app/node_modules/sails-      postgresql/node_modules/pg.js/lib/connection.js:534:11)
at Connection.parseMessage (/home/valentina/workspace/xtens-app/node_modules/sails-postgresql/node_modules/pg.js/lib/connection.js:361:17)
at Socket.<anonymous> (/home/valentina/workspace/xtens-app/node_modules/sails-postgresql/node_modules/pg.js/lib/connection.js:105:22)
at Socket.emit (events.js:95:17)
at Socket.<anonymous> (_stream_readable.js:764:14)
at Socket.emit (events.js:92:17)
at emitReadable_ (_stream_readable.js:426:10)
at emitReadable (_stream_readable.js:422:5)
at readableAddChunk (_stream_readable.js:165:9)
at Socket.Readable.push (_stream_readable.js:127:10)

Details:  error: relation "group_operators__operator_groups" does not exist

I'm using sails 0.10.5, sails-postgresql ^0.10.8 and postgresql 9.3.4.

Should I create the join table? What is the problem? The join table is empty, so the association doesn't exist.

Thank you!

Valentina