Rails ActiveRecord select method adds the ability to receive hash values

Rails ActiveRecord #select is one of the fundamental methods to fetch the model attributes when querying on a table.

It is served in two ways:

It takes a block, that gets executed like Array#select. For e.g.,
```
Post.all.select{ |post| post.title.include?('Rails') }
```
The above code will first fetch all the posts from the database. Then the select command will iterate through all the posts and extract the ones which contain the title Rails. The above code returns an array of post objects.

When used on a model, it works as the SELECT statement for the query to fetch particular fields. For e.g.,

Post.select(:title, :description)

=> SELECT \"posts\".\"title\", \"posts\".\"description\" FROM \"posts\"

When working on multiple tables using joins, or includes, we can select fields of different tables as below:

Post
  .joins(:comments)
  .select(
    'posts.id as post_id, posts.title as post_tile,
    comments.id as comment_id, comments.body as comment_body'
  )

Before

As seen above, we had to write and pass an SQL statement to the select method when working with associated tables. This is not similar to what we do when using the where clause.

Post
  .joins(:comments)
  .where(
    comments: {
      author_id: 1
    }
  )

# OR

Post
  .joins(:comments)
  .where(
    'comments.author_id = ?', 1
  )

The where method accepts a hash and can process SQL statements too. If we try to pass a hash to the select method, it will raise an error as below:

Post
  .joins(:comments)
  .select(
    { comments: { id: :comment_id, body: :comment_body } }
  )

Arel::Visitors::UnsupportedVisitError: Unsupported argument type: Hash. Construct an Arel node instead.

After

To reduce efforts of writing raw SQL statements and keeping things consistent across Rails query methods, ActiveRecord::QueryMethods#select** allows accepting hash.

The hash values can be an array of field attributes of the associated model as below:

Post
  .joins(:comments)
  .select(
    { comments: [:id, :body] }
  )

We can also pass a hash of fields and aliases as below:

Post
  .joins(:comments)
  .select(
    { comments: {id: :comment_id, body: :comment_body } }
  )

To know more about this feature, please check this Pull Request.