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

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

CommentMapper.belongsTo(UserMapper, {
  // comment.user_id points to user.id
  foreignKey: 'user_id'
  // user records will be attached to comment records at "comment.user"
  localField: 'user'
});
CommentMapper.belongsTo(PostMapper, {
  // comment.post_id points to post.id
  foreignKey: 'post_id'
  // post records will be attached to comment records at "comment.post"
  localField: 'post'
});

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