How to fetch only specified fields of model with DataMapper?

问题

I cant find the way to fetch only necessary fields of model in certain context. Let`s say i have a huge model which has about 30 fields (properties). But for a search purpose i need only couple of them.

Example:

class Foo
  include DataMapper::Resource

  property :id, Serial
  property :title, String, :length => 256
  property :description, Text
  property :url, String, :length => 4096
  property :city, String, :length => 64
  property :country, String, :length => 64
  property :state, String, :length => 64
  property :created_at, Time
  property :updated_at, Time
  property :expires_at, Time
  ... etc fields ...
end

So, for the front-end (web application) most of that fields are used. But for search i need let`s say only :title, :city, :state. Its easy to query data, like that

items = Foo.all(:title.like => 'Some stuff')

And then i need to pack fetched data into JSON. As far as i know, there is a module for DataMapper called dm-serialize which handles all that operations.

Finally, i do the package for output:

response = {:error => 0, :count => items.length, :data => items}
response.to_json

But the output items have all the fields while i need to get only some of them. For some reason lazy loading does not work.

The question is: How to specify model fields you are going to select?

回答1:

Foo.all(:fields=>[:title, :city, :state])

回答2:

Been stumbling over this, too.
Also, provide the :only option to #to_json method, otherwise it will lazy load the ones not yet fetched.

items.to_json(:only => [:title, :city, :state])

You have to build the json response yourself; otherwise lazy fetches of the other fields will occur.

来源：https://stackoverflow.com/questions/1967017/how-to-fetch-only-specified-fields-of-model-with-datamapper

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ruby

datamapper