Eager Finder SQL

EagerFinderSql allows custom SQL to be specified when doing eager loading of associations through the :include option to find. This allows for purpose-constructed queries to be used and still result in a fully linked object model.

Background

ActiveRecord constructs SQL to satisfy the requirements of a find request. Associations allow for customized SQL to be specified, using the :finder_sql option, but this has not been available when performing eager loading using the :include option. The result is that a standardized query is constructed to bring in the associated tables using LEFT OUTER JOIN. For some queries, this can be result in expensive queries and potentially very large result sets.

Custom SQL

EagerFinderSql addresses this problem by allowing :finder_sql to be added to find options when the :include option is also specified. Columns in the result set are mapped to attributes in the resulting object model through the :column_mapping option. The :column_mapping option specifies a hash containing the following entries:

  • primary_key – indicates the column alias associated with the classes id attribute.
  • columns – is a hash of attribute to column alias associations.
  • associations – is a hash of association mappings for each directly included model. The key for each entry is the name of the association, while the value is a hash similar to the hash for the parent class.

Example

Consider the following query relating authors with many books:

Author.find(:all, :include => :books)

This would likely produce the following SQL:

SELECT
    authors.id AS t0_c0, authors.name AS t0_c1,
    books.id AS t1_c0, books.author_id AS t1_c1, books.title AS t1_c2
  FROM authors
  LEFT OUTER JOIN books ON books.author_id = authors.id

The query might be written with books as the driver, rather than authors as follows:

 SELECT
    authors.id, authors.name,
    books.id AS book_id, books.name AS book_name
  FROM books
  JOIN authors ON authors.id = books.author_id

The Rails query would then be written as follows:

Author.find(:all, :include => books,
           :finder_sql => "
                SELECT
                    authors.id, authors.name,
                    books.id AS book_id, books.name AS book_name
                  FROM books
                  JOIN authors ON authors.id = books.author_id".
           :column_mapping => {
             :primary_key => 'id',
             :columns => {
                  'id'  => 'id',
                  'name => 'name'
              },
              :associations => {
                :books => {
                  :primary_key => book_id,
                  :columns => {
                      'id'        => 'book_id',
                      'author_id' => 'id',
                      'name'      => 'book_name
                  }
                }
              }
            })

This is more verbose, but allows for absolute control of the SQL used to return results across multiple model associations. A more complicated example would be the following:

QUERY = "
SELECT
    a.id AS author_id,
    a.name AS author_name,
    p.id AS post_id,
    p.title AS post_title,
    p.body AS post_body,
    p.type AS post_type,
    c.id AS comment_id,
    c.body AS comment_body,
    c.type AS comment_type
  FROM authors a
  LEFT JOIN posts p ON p.author_id = a.id
  LEFT JOIN comments c ON c.post_id = p.id"

MAPPING = {
  :primary_key => 'author_id',
  :columns => {
    'id' => 'author_id',
    'name' => 'author_name'
  },                         
  :associations=> {
    :posts => {
      :primary_key  => 'post_id',
      :columns => {
        'id' => 'post_id',
        'title' => 'post_title',
        'author_id' => 'author_id',
        'body' => 'post_pody',
        'type' => 'post_type'
      },
      :associations => {
        :comments => {
          :primary_key  => 'comment_id',
          :columns => {
            'id' => 'comment_id',
            'post_id' => 'post_id',
            'author_id' => 'author_id',
            'body' => 'comment_body',
            'type' => 'comment_type'
          }
        }
      }
    }
  }
}
Author.find(:all,
            :include=>{:posts=>:comments},
            :order=>"authors.id",
            :finder_sql => QUERY,
            :column_mapping => MAPPING)

This example shows the recursive nature of associations. It can also be used to render deeper structures that may reference the same model multiple times, such as the following:

Firm.find(:all,
          :include=>{:account=>{:firm=>:account}},
          :order=>"companies.id")

(Here, the query and mapping is left to the reader; or, you can look at the unit test which documents many more possibilities). Install using

script/plugin install svn://rubyforge.org/var/svn/eagerfindersql

The plugin is managed on RubyForge. Rdoc is available.