Merge Sep 23 2013

Rails has a functionality that I use a lot but I think it is not as widespread as it should. I’m talking about the ActiveRecord’s merge method. Probably, the reason why this method is not widely used is because there is a method with the same name in the Hash class and that causes some confusion.

Hash’s merge

The Hash’s merge method merges two hashes and returns a third hash. Example:

h1 = {a: 'cat', b: 'dog'}
h2 = {c: 'bird'}
h3 = h1.merge(h2)
# => {:a=>"cat", :b=>"dog", :c=>"bird"}

This is a method available on the Ruby Core.

ActiveRecord’s merge

The ActiveRecord’s merge, instead, helps to build queries that involve two or more models that need to work with defined scopes. What it does is merging scopes prior to perform the query.

Example:

Let’s build two models, Client wihch has many Products, each one with some scopes.

# app/models/client.rb
class Client < ActiveRecord::Base
  has_many :products

  scope :recently_activated, -> { where 'activated_at > ?', 1.month.ago }
end

# app/models/product.rb
class Product < ActiveRecord::Base
  belongs_to :client

  scope :expensive,  -> { where 'products.price > 1000' }
  scope :small,      -> { where 'products.size <= 99' }
  scope :medium,     -> { where 'products.size > 99 AND products.size < 999' }
  scope :big,        -> { where 'products.size >= 999' }
  scope :available,  -> { where available: true }
end

Now, if you want to know all the recently activated clients who bought expensive medium available products, you have to build your query this way:

Client.recently_activated.joins(:products).
  where('products.price > 1000 AND products.size > 99 AND products.size < 999 AND products.available = ?', true)

Or, using the merge method.

Client.recently_activated.joins(:products).merge(Product.expensive.medium.available)

The last code is not only easier to build and easier to read, it also follows better a DRY concept. If, for any reason, the definition of ‘medium’ changes, you only have to change it in one place, all your queries remain intact.

It is also possible to use merge inside a scope. Example:

# app/models/client.rb
class Client < ActiveRecord::Base
  has_many :products

  scope :recently_activated, -> { where 'activated_at > ?', 1.month.ago }
  scope :with_expensive_products, -> { joins(:products).merge(Product.expensive) }
end

Quite useful, right?