hasOne (relatedMapper, opts) {
    return hasOne(relatedMapper, opts)(this)
  }
Defines a hasOne relationship. Only useful if you're managing your Mappers
manually and not using a {@link Container} or {@link DataStore} component.

@example
UserMapper.hasOne(ProfileMapper, {
  // profile.user_id points to user.id
  foreignKey: 'user_id'
  // profile records will be attached to user records at "user.profile"
  localField: 'profile'
});

@method Mapper#hasOne
@see http://www.js-data.io/v3.0/docs/relations
@since 3.0.0