static findBy(field, value, callback) {

    let db = this.prototype.db;
    let query = {};
    query[field] = value;

    return new Composer(this)
      .where(query)
      .end((err, models) => {

        if (!err && !models.length) {
          let err = new Error(`Could not find ${this.name} with ${field} "${value}".`);
          err.notFound = true;
          return callback(err);
        }

        callback(err, models[0]);

      });

  }
Finds a model with a provided field, value pair. Returns the first found.
@param {string} field Name of the field
@param {any} value Value of the named field to compare against
@param {function({Error} err, {Nodal.Model} model)} callback The callback to execute upon completion