Preload Model Associations On Find

Question

Right now I am trying to call my model, and spit out its results into json/xml format. My only problem is, my database associations aren't being loaded or queried.

Normally, I can just run this

@campaign = Campaign.find(:all)

Then get the number of hits by calling, @campaign[0].hits through the has_many :hits.

But if you debug the output, it only calls the columns on the table. How would you go about having it put it alongside your query?

In example:

  <campaign>
    <category>website</category>
    <created-at type="timestamp">2009-01-24 14:49:02 -0800</created-at>
    <end-at type="date">2009-01-24</end-at>
    <id type="integer">14</id>
    <is-watched type="integer">1</is-watched>
    <name>Lets</name>
    <slug>c5334415da5c89384e42ce6d72609dda</slug>
    <start-at type="date">2009-01-24</start-at>
    <user-id type="integer">5</user-id>
  </campaign>

Then having it instead add another column, but witht he number of hits.

  <campaign>
    <category>website</category>
    <created-at type="timestamp">2009-01-24 14:49:02 -0800</created-at>
    <end-at type="date">2009-01-24</end-at>
    <id type="integer">14</id>
    <is-watched type="integer">1</is-watched>
    <name>Lets</name>
    <slug>c5334415da5c89384e42ce6d72609dda</slug>
    <start-at type="date">2009-01-24</start-at>
    <user-id type="integer">5</user-id>
    <hits type="integer">123412</hits>
  </campaign>

Answer 1

The ActiveRecord find() family takes an :include option which allows you to eager load your associatons.

So all you need to do is:

@campaign = Campaign.find(:all, :include => :hits)

The above will eager load your database calls so that accessing each index [0]. [1], etc wont issue their own SELECT calls. When you call

@campaign[0].to_json

Then it will NOT include any associations, such as "hits" to do that then you also need to :include it on the to_json call, e.g.

@campaign[0].to_json(:include => :hits)