arel helpers

arel-helpers

Useful tools to help construct database queries with ActiveRecord and Arel.

Installation

gem install arel-helpers

Usage

require 'arel-helpers'

ArelTable Helper

Usually you’d reference database columns in Arel via the #arel_table method on your ActiveRecord models. For example:

class Post < ActiveRecord::Base
  ...
end

Post.where(Post.arel_table[:id].eq(1))

Typing “.arel_table” over and over again can get pretty old and make constructing queries unnecessarily verbose. Try using the ArelTable helper to clean things up a bit:

class Post < ActiveRecord::Base
  include ArelHelpers::ArelTable
  ...
end

Post.where(Post[:id].eq(1))

JoinAssociation Helper

Using pure Arel is one of the only ways to do an outer join with ActiveRecord. For example, let’s say we have these two models:

class Author < ActiveRecord::Base
  has_many :posts

  # attribute id
  # attribute username
end

class Post < ActiveRecord::Base
  belongs_to :author
  has_many :comments

  # attribute id
  # attribute author_id
  # attribute subject
end

class Comment < ActiveRecord::Base
  belongs_to :post
  belongs_to :author

  # attribute id
  # attribute post_id
  # attribute author_id
end

A join between posts and comments might look like this:

Post.joins(:comments)

ActiveRecord introspects the association between posts and comments and automatically chooses the right columns to use in the join conditions.

Things start to get messy however if you want to do an outer join instead of the default inner join. Your query might look like this:

Post.joins(
  Post.arel_table.join(Comment.arel_table, Arel::Nodes::OuterJoin)
    .on(Post[:id].eq(Comment[:post_id]))
    .join_sources
)

Such verbose. Much code. Very bloat. Wow. We’ve lost all the awesome association introspection that ActiveRecord would otherwise have given us. Enter ArelHelpers.join_association:

Post.joins(
  ArelHelpers.join_association(Post, :comments, Arel::Nodes::OuterJoin)
)

Easy peasy.

Note that pretty much anything you can pass to ActiveRecord’s #join method you can also pass to #join_association’s second argument. For example, you can pass a hash to indicate a set of nested associations:

Post.joins(
  ArelHelpers.join_association(Post, { comments: :author })
)

This might execute the following query:

SELECT "posts".*
FROM "posts"
INNER JOIN "comments" ON "comments"."post_id" = "posts"."id"
INNER JOIN "authors" ON "authors"."id" = "comments"."author_id"

#join_association also allows you to customize the join conditions via a block:

Post.joins(
  ArelHelpers.join_association(Post, :comments, Arel::Nodes::OuterJoin) do |assoc_name, join_conditions|
    join_conditions.and(Post[:author_id].eq(4))
  end
)

But wait, there’s more! Include the ArelHelpers::JoinAssociation concern into your models to have access to the join_association method directly from the model’s class:

include ArelHelpers::JoinAssociation

Post.joins(
  Post.join_association(:comments, Arel::Nodes::OuterJoin) do |assoc_name, join_conditions|
    join_conditions.and(Post[:author_id].eq(4))
  end
)

Query Builders

ArelHelpers also contains a very simple class that’s designed to provide a light framework for constructing queries using the builder pattern. For example, let’s write a class that encapsulates generating queries for blog posts:

class PostQueryBuilder < ArelHelpers::QueryBuilder
  def initialize(query = nil)
    # whatever you want your initial query to be
    super(query || post.unscoped)
  end

  def with_title_matching(title)
    reflect(
      query.where(post[:title].matches("%#{title}%"))
    )
  end

  def with_comments_by(usernames)
    reflect(
      query
        .joins(comments: :author)
        .where(author[:username].in(usernames))
    )
  end

  def since_yesterday
    reflect(
      query.where(post[:created_at].gteq(Date.yesterday))
    )
  end

  private

  def author
    Author
  end

  def post
    Post
  end
end

The #reflect method creates a new instance of PostQueryBuilder, copies the query into it and returns the new query builder instance. This allows you to chain your method calls:

PostQueryBuilder.new
  .with_comments_by(['camertron', 'catwithtail'])
  .with_title_matching("arel rocks")
  .since_yesterday

Conditional reflections

If you have parts of a query that should only be added under certain conditions you can return reflect(query) from your method. E.g:

  def with_comments_by(usernames)
    if usernames
      reflect(
        query.where(post[:title].matches("%#{title}%"))
      )
    else
      reflect(query)
    end
  end

This can become repetitive, and as an alternative you can choose to prepend not_nil to your method definition:

  class PostQueryBuilder < ArelHelpers::QueryBuilder
    not_nil def with_comments_by(usernames)
      reflect(query.where(post[:title].matches("%#{title}%"))) if usernames
    end
  end

Requirements

Requires ActiveRecord >= 3.1.0, < 8. Depends on SQLite for testing purposes.

Running Tests

bundle exec rspec

Authors

• Cameron C. Dutro: http://github.com/camertron