Rails Joins and include columns from joins table

Question

Use select() to name the columns you want. At least this works in Rails 3.0.9.

Background: my application has a primary table named :rights. I wanted to be able to ascribe a tag and color to a given :right record so I could easily pick it out of an index listing. This doesn’t cleanly fit the Rails picture of associated records; most :rights will never be tagged, and the tags are completely arbitrary (user input via tag/edit).

I could try duplicating the tag data in the :right record, but that violates normal form. Or I could try querying :tags for each :right record, but that is a painfully inefficient approach. I want to be able to join the tables.

MySQL console shows:

mysql> describe rights;
+------------+---------------+------+-----+---------+----------------+
| Field      | Type          | Null | Key | Default | Extra          |
+------------+---------------+------+-----+---------+----------------+
| id         | int(11)       | NO   | PRI | NULL    | auto_increment |

  ...

| Tagid      | int(11)       | YES  |     | NULL    |                |
+------------+---------------+------+-----+---------+----------------+

mysql> describe tags;
+------------+--------------+------+-----+---------+----------------+
| Field      | Type         | Null | Key | Default | Extra          |
+------------+--------------+------+-----+---------+----------------+
| id         | int(11)      | NO   | PRI | NULL    | auto_increment |
| TagName    | varchar(255) | YES  |     | NULL    |                |
| TagColor   | varchar(255) | YES  |     | NULL    |                |
| created_at | datetime     | YES  |     | NULL    |                |
| updated_at | datetime     | YES  |     | NULL    |                |
+------------+--------------+------+-----+---------+----------------+

I am going to use TagName and TagColor in views/rights/index.html.erb, so I want the rights controller to include those columns in the @rights object it passes to the view. Since not every :right has a :tag, I want to use an outer join:

@rights = Right.joins("LEFT OUTER JOIN tags ON rights.Tagid = tags.id")

But, as everyone has found, this alone doesn’t work: a block reference to TagName produces a server error. However, if I add a select at the end, all is well:

@rights = Right.joins("LEFT OUTER JOIN tags ON rights.Tagid = tags.id").select("rights.*,tags.TagName as TagName,tags.TagColor as TagColor")

Note added 6/7/13: the select clause does not require aliases – this works too:

.select("rights.*,tags.TagName,tags.TagColor")

Now I can reference TagName and TagColor in my view:

<% @rights.each do |right| %>
  <tr ALIGN=Left <%=
  # color background if this is tagged
  " BGCOLOR=#{right.TagColor}" if right.TagColor
  %> > ...
<% end %>

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