join_through.js

var knex = require('knex')({
        client: 'pg',
        debug: true,
        connection: {
            host: 'localhost',
            //user: 'your_database_user',
            //password: 'your_database_password',
            database: 'ecare_dump_db',
            charset: 'utf8'
        }
    }),
    bookshelf = require('bookshelf')(knex),

    Region = bookshelf.Model.extend({
        tableName: 'regions',

        published_documents: function () {
            return this.belongsToMany(PublishedDocument).through(PublishedDocumentRegion, 'region_id', 'published_document_id');
        }
    }),

    PublishedDocumentRegion = bookshelf.Model.extend({
        tableName: 'published_document_regions'
    }),

    PublishedDocument = bookshelf.Model.extend({
        tableName: 'published_documents',

        regions: function () {
            return this.belongsToMany(Region).through(PublishedDocumentRegion, 'published_document_id', 'region_id');
        }
    });


new PublishedDocument().regions().query({where: {'regions.id': 1}}).fetch({withRelated: 'regions'}).then(function (collection) {
   console.log("length: ",collection.length);
});