predicate_scope

predicate_scope

Have you ever written code like this?

class User < ActiveRecord::Base
  scope :active, -> { where(deleted: false, state: "confirmed") }

  def active?
    !deleted && state == "confirmed"
  end
end

And noticed that the definition of an "active" user is duplicated? What if the definition changes, and you forget to update both places?

With predicate_scope you can write it once:

class User < ActiveRecord::Base
  include PredicateScope

  predicate_scope :active, -> { where(deleted: false, state: "confirmed") }
end

This defines two methods:

• A User.active scope, as if you had used scope like normal.
• A predicate instance method User#active?, which behaves just like the hand-written version of active? in the original example:

user = User.new(deleted: false, state: "confirmed")
user.active? # true
user.state = "unconfirmed"
user.active? # false

The predicate method checks the conditions of the scope against the instance in memory, without querying the database. Again, just like the hand-written active?.

Implementation

In addition to predicate_scope, this gem also defines a instance method #satisfies_conditions_of?. This takes an ActiveRecord::Relation and returns whether the instance it is called on satisfies its conditions. The predicate methods defined by predicate_scope call satisfies_conditions_of? with the relation from the scope. So in the above example user.active? is implemented as user.satisfies_conditions_of?(User.active).

satisfies_conditions_of? extracts the Arel abstract syntax tree (AST) from the given ActiveRecord::Relation and interprets its conditions as Ruby predicates. These predicates are evaluated against the instance.

Limitations

Not all Arel operations are implemented. If you define a predicate_scope that uses an unsupported operation PredicateScope::Errors::UnsupportedOperation will be raised when the predicate method is called. PRs to implement additional operations are appreciated!

Installation

Add this line to your application's Gemfile:

gem 'predicate_scope'

And then execute:

$ bundle

Or install it yourself as:

$ gem install predicate_scope

In any models where you want to use predicate_scope add

include PredicateScope

Contributing

Bug reports and pull requests are welcome on GitHub at https://github.com/CJStadler/predicate_scope.

License

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