CalculatedAttributes

Automatically add calculated attributes from accessory select queries to ActiveRecord models.

Installation

Add this line to your application's Gemfile:

gem 'calculated_attributes'

And then execute:

$ bundle

Or install it yourself as:

$ gem install calculated_attributes

Usage

Add each calculated attribute to your model using the calculated keyword. It accepts two parameters: a symbol representing the name of the calculated attribute, and a lambda containing a string to calculate the attribute. The lambda can accept arguments.

For example, if we have two models, Post and Comment, and Comment has a post_id attribute, we might write the following code to add a comments count to each Post record in a relation:

class Post < ActiveRecord::Base
...
  calculated :comments_count, -> { "select count(*) from comments where comments.post_id = posts.id" }
  calculated :comments_count_by_user, ->(user) { ["select count(*) from comments where comments.post_id = posts.id and posts.user_id = '%s'", user.id] }
...
end

Then, the comments count may be accessed as follows:

Post.scoped.calculated(:comments_count).first.comments_count
Post.scoped.calculated(comments_count_by_user: user).first.comments_count_by_user
#=> 5

Multiple calculated attributes may be attached to each model. If we add a Tag model that also has a post_id, we can update the Post model as following:

class Post < ActiveRecord::Base
...
  calculated :comments_count, -> { "select count(*) from comments where comments.post_id = posts.id" }
  calculated :tags_count, -> { "select count(*) from tags where tags.post_id = posts.id" }
...
end

And then access both the comments_count and tags_count like so:

post = Post.scoped.calculated(:comments_count, :tags_count).first
post.comments_count
#=> 5
post.tags_count
#=> 2

Note that you must call calculated on a relation in order to get the desired result. Post.calculated(:comments_count) will give you the currently defined lambda for calculating the comments count. Post.scoped.calculated(:comments_count) (Rails 3) or Post.all.calculated(:comments_count) (Rails 4) will give you an ActiveRecord relation including the calculated attribute.

You may also use the calculated method on a single model instance, like so:

Post.first.calculated(:comments_count).comments_count
#=> 5

Post.first.calculated(comments_count_by_user: user).comments_count_by_user
#=> 0

If you have defined a calculated method, results of that method will be returned rather than throwing a method missing error even if you don't explicitly use the calculated() call on the instance:

Post.first.comments_count
#=> 5
Post.first.comments_count_by_user(user)
#=> 0

If you like, you may define calculated lambdas using Arel syntax:

class Post < ActiveRecord::Base
...
  calculated :comments_count, -> { Comment.select(Arel::Nodes::NamedFunction.new("COUNT", [Comment.arel_table[:id]])).where(Comment.arel_table[:post_id].eq(Post.arel_table[:id])) }
...
end

Known Issues

In Rails 4.x, you cannot call count on a relation with calculated attributes, e.g.

Post.scoped.calculated(:comments_count).count

will error. This is because of an ActiveRecord issue that does not permit Arel nodes in the count method.

Contributing

1. Fork it ( https://github.com/aha-app/calculated_attributes/fork )
2. Create your feature branch (git checkout -b my-new-feature)
3. Commit your changes (git commit -am 'Add some feature')
4. Push to the branch (git push origin my-new-feature)
5. Create a new Pull Request

Credits

Written by Zach Schneider based on ideas from Chris Waters.