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diff --git a/README.markdown b/README.markdown index 75b0b3ee0b..f12aa67498 100644 --- a/README.markdown +++ b/README.markdown @@ -160,7 +160,7 @@ products. #### Complex Joins -Where Arel really shines in its ability to handle complex joins and aggregations. As a first example, let's consider an "adjacency list", a tree represented in a table. Suppose we have a table `comments`, representing a threaded discussion: +Where Arel really shines is in its ability to handle complex joins and aggregations. As a first example, let's consider an "adjacency list", a tree represented in a table. Suppose we have a table `comments`, representing a threaded discussion: ```ruby comments = Arel::Table.new(:comments) @@ -172,16 +172,17 @@ And this table has the following attributes: # [:id, :body, :parent_id] ``` -The `parent_id` column is a foreign key from the `comments` table to itself. Now, joining a table to itself requires aliasing in SQL. In fact, you may alias in Arel as well: +The `parent_id` column is a foreign key from the `comments` table to itself. +Joining a table to itself requires aliasing in SQL. This aliasing can be handled from Arel as below: ```ruby replies = comments.alias comments_with_replies = \ - comments.join(replies).on(replies[:parent_id].eq(comments[:id])) -# => SELECT * FROM comments INNER JOIN comments AS comments_2 WHERE comments_2.parent_id = comments.id + comments.join(replies).on(replies[:parent_id].eq(comments[:id])).where(comments[:id].eq(1)) +# => SELECT * FROM comments INNER JOIN comments AS comments_2 WHERE comments_2.parent_id = comments.id AND comments.id = 1 ``` -This will return the first comment's reply's body. +This will return the reply for the first comment. [Common Table Expresssions(CTE)](https://en.wikipedia.org/wiki/Common_table_expressions#Common_table_expression) support via: |