ArelToolkit contains parsing, querying, modifying, optimisations, extensions and more for Arel.

gem install arel_toolkit -v 0.4.1





Add this line to your application's Gemfile:

gem 'arel_toolkit'

And then execute:

$ bundle

Or install it yourself as:

$ gem install arel_toolkit

Sql to Arel

Convert your (PostgreSQL) SQL into an Arel AST.

[1] > sql = 'SELECT id FROM users'
=> "SELECT id FROM users;"
[2] > arel = Arel.sql_to_arel(sql)
=> #<Arel::SelectManager:0x00007fe4e39823d8>
[3] > arel.to_sql
=> "SELECT \"id\" FROM \"users\""

Enhanced Arel AST

Arel.enhance(arel) adds additional information and helper methods to the existing Arel AST. This allows for mutating the AST, adding contextual information to the AST and querying for nodes. Some examples:

Query for Arel nodes with certain properties
arel =, :public).where(id: 1).arel
enhanced_arel = Arel.enhance(arel)
enhanced_arel.query(class: Arel::Table).each { ... }
Query for Arel nodes with an enhanced context

An Arel::Table is used in multiple different places inside the AST, and those locations will give the Arel::Table a different meaning. Used within a projection (column_reference) like SELECT has a different meaning than within a from SELECT * FROM posts (range_variable). The following example results in Arel::Table nodes where the object is used in the context of referencing a column:

enhanced_arel.query(class: Arel::Table, context: { column_reference: true }).each { ... }
Get an Arel node at a certain path
enhanced_arel.child_at_path(['ast', 'cores', 0, 'projections', 1]).object
=> #<struct Arel::Attributes::Attribute>
Replace or remove nodes without modifying the original arel

remove and replace allow for modifications to the Arel AST. The changes are aplied to a new copy of the AST, making sure the original AST is not touched.

enhanced_arel.child_at_path(['ast', 'cores', 0, 'projections', 1]).replace(Post.arel_table[:content])
enhanced_arel.child_at_path(['ast', 'cores', 0, 'projections', 0]).remove
=> SELECT "posts"."content" FROM "posts" WHERE "posts"."id" = $1


Creating Arel from SQL and enhancing Arel is just the beginning, where this gem really shines is the ability to modify Arel ASTs using middleware.

Middleware sits between ActiveRecord and the database, it allows you to alter the Arel (the SQL query) before it's send to the database. Multiple middlewares are supported by passing the results from a finished middleware to the next. Next to the arel object, a context object is used that acts as a intermediate storage between middlewares.

The middleware works out of the box in combination with Rails. If using ActiveRecord standalone you need to run the following after setting up the database connection:



Create middleware which can be any Ruby object as long as it responds to call. Middleware accepts 2 or 3 arguments, context is optional. Calling .call on next_middleware invokes the next middleware in the chain, returning the response from the database.

In this example, we're creating a middleware that will reorder any query. Next to reordering, we're adding an additional middleware that prints out the result of the reorder middleware.

class ReorderMiddleware
  def, next_middleware)
    enhanced_arel = Arel.enhance(arel)
    enhanced_arel.query(class: Arel::Nodes::SelectStatement).each do |node|
      arel_table = node.child_at_path(['cores', 0, 'source', 'left']).object

    new_arel = arel.order(Post.arel_table[:id].asc)

class LoggingMiddleware
  def, next_middleware, context)
    puts "User executing query: `#{context[:current_user_id]}`"
    puts "Original SQL: `#{context[:original_sql]}`"
    puts "Modified SQL: `#{arel.to_sql}`"

Now that we've defined our middelwares, it's time to see them in action:

[1] > Arel.middleware.apply([ReorderMiddleware, LoggingMiddleware]).context(current_user_id: 1) { Post.all.load }
User executing query: `1`
Original SQL: `SELECT "posts".* FROM "posts"`
Modified SQL: `SELECT "posts".* FROM "posts" ORDER BY "posts"."id" ASC`
Post Load (4.1ms)  SELECT "posts".* FROM "posts" ORDER BY "posts"."id" ASC
=> []

This gem ships with a couple of middelware methods that allow you to fine-tune what and when to apply middelware.

  • Arel.middleware.apply([SomeMiddleware]) { ... }
  • Arel.middleware.only([OnlyMe]) { ... }
  • Arel.middleware.none { ... }
  • Arel.middleware.except(RemoveMe) { ... }
  • Arel.middleware.insert_before(RunBefore, ThisMiddleware) { ... }
  • Arel.middleware.insert_after(RunAfter, ThisMiddleware) { ... }


This gem aims to have full support for PostgreSQL's SQL. In order to do so, it needs to add missing Arel nodes and extends the existing visitors. A full list of extensions on Arel can be found here: lib/arel/extensions.


After checking out the repo, run bin/setup to install dependencies. Then, run rake spec to run the tests. You can also run bin/console for an interactive prompt that will allow you to experiment.

To install this gem onto your local machine, run bundle exec rake install. To release a new version, update the version number in version.rb, and then run bundle exec rake release, which will create a git tag for the version, push git commits and tags, and push the .gem file to


Bug reports and pull requests are welcome on GitHub at This project is intended to be a safe, welcoming space for collaboration, and contributors are expected to adhere to the Contributor Covenant code of conduct.


The gem is available as open source under the terms of the MIT License.

Code of Conduct

Everyone interacting in the ArelToolkit project’s codebases, issue trackers, chat rooms and mailing lists is expected to follow the code of conduct.