Rails 3: Rendering certain attributes of an array of objects in JSON

Question

I'm trying to pass some data to javascript in my view. I only want certain attributes of the objects in the array.

The json gem doesn't appear to support the :only option. I tried to use ActiveSupport::JSON

<script>
test1=<%=raw ActiveSupport::JSON.encode(@sectionDatas.values, :only => [ :left, :width ])%>;
</script>

but that ignores the :only and prints the whole object.

Then I thought I would be clever and take the render method from the controller:

test2=<%=raw render :json => @sections.as_json(:only => [:left, :width])%>

but I get Nil:Nilclass errors.

I also tried to put this in my model and run to_json:

include ActiveModel::Serialization::JSON

def attributes
  @attributes ||= {'left' => 0, 'width'=>0}
end

Again, this ignores the attributes method and serializes the whole object.

Surely this must be simple. What am I missing?

John · Accepted Answer · 2011-03-21 23:11:17Z

3

Assuming the objects in the array are instances of ActiveRecord::Base or include ActiveModel::Serialization::JSON:

test2=<%=raw @sections.to_json(:only => [:left, :width]) %>

answered Mar 21, 2011 at 23:11

John

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Community · Accepted Answer · 2017-05-23 12:23:08Z

2

You can filter out the columns you don't need when you get objects from the db with select.

Item.find( :all, :select => 'DISTINCT fieldname' )

Of course, this is not the Rails3 way. Here that is:

Model.select(attribute)

Update

If you want to have the original array of objects and json but the json with filtered attributes you will need to override to_json:

This post explains how to do that:

How to override to_json in Rails?

edited May 23, 2017 at 12:23

CommunityBot

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answered Mar 21, 2011 at 22:27

Sam 山

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4 Comments

bcoughlan Over a year ago

Won't that cause an extra query to the database for data I already have?

Sam 山 Over a year ago

I was assuming that you would replace the it with the code I suggested.

bcoughlan Over a year ago

Yep but I need the full object too, don't see the use in doing an extra query for data I already have. Also I can't find any docs on .select, is it part of ActiveRecord?

Sam 山 Over a year ago

you need to override to_json in your model then. Check the update.

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