belongsTo = function belongsTo(related, opts) {
  return function (mapper) {
    Relation.belongsTo(related, opts).assignTo(mapper);
  };
}
BelongsTo relation decorator. You probably won't use this directly.

@name module:js-data.belongsTo
@method
@param {Mapper} related The relation the target belongs to.
@param {object} opts Configuration options.
@param {string} opts.foreignKey The field that holds the primary key of the
related record.
@param {string} opts.localField The field that holds a reference to the
related record object.
@returns {Function} Invocation function, which accepts the target as the only
parameter.