Among other great features of ActiveRecord in Rails, I find accepts_nested_attributes_for is one of the most useful. It was introduced in Rails 2.3 and allows you to save attributes on associated records through the parent.
If you are not familiar with accepts_nested_attributes_for method, I suggest you read the official documentation before reading this article.
There are some options that you can pass to accepts_nested_attributes_for method. Lets quickly go through them.
-
:update_only- If you want to update the existing record only and not create new one, you can passupdate_only: trueto the method. This works only for one-to-one association. -
:reject_if- You can pass a proc to:reject_ifoption to reject the nested attributes if they don’t meet the criteria. For example, if you want to reject the nested attributes if thenameattribute is blank, you can do it like this:
accepts_nested_attributes_for :comments, reject_if: proc { |attributes| attributes['name'].blank? }
-
:allow_destroy- If you want to destroy the associated record by passing_destroy: 1in the nested attributes, you can passallow_destroy: trueto the method. -
:limit- If you want to limit the number of nested attributes that can be submitted.
But what if you want to want to identify the nested model by some other attribute than id? Let me show you an example.
# app/models/work.rb
class Work < ApplicationRecord
belongs_to :inventory, autosave: true
accepts_nested_attributes_for :inventory
end
# app/models/inventory.rb
class Inventory < ApplicationRecord
belongs_to :work
validates :barcode_data, uniqueness: true, allow_blank: true
end
# == Schema Information
#
# Table name: inventories
#
# id :bigint not null, primary key
# barcode_data :string
#
# Indexes
#
# index_inventories_on_barcode_data (barcode_data) UNIQUE
Example above consists of two models where Work has an Inventory. Inventory model has a barcode_data column which is unique, but can be blank. To make this example easier, I didn’t include other Inventory attributes, some of them can also can identify it. There are also tables and models to track barcode history, but they are also not relevant to this example.
Now, I want to create a new work with inventory and barcode_data. Below is the list of params that will be passed to Work.create method.
{
work: {
inventory_attributes: {
barcode_data: '1234567890'
}
}
}
For the first time, when the barcode is not present in the database, it will create a new barcode record with data: '1234567890'. When I pass the same set of params again to Work.create method I will get a ActiveRecord::RecordNotUnique error because the inventory with barcode_data: '1234567890' already exists in the database, which makes total sense because accepts_nested_attributes_for uses id to identify the persisted nested model.
Instead of getting error I want to find the existing inventory record by barcode_data and update it. To achieve this, I need to override the inventory_attributes= method in the Work model.
# app/models/work.rb
class Work < ApplicationRecord
belongs_to :inventory, autosave: true
accepts_nested_attributes_for :inventory
def inventory_attributes=(inventory_attributes)
barcode_data = inventory_attributes["barcode_data"]
if (inventory = Inventory.find_by(barcode_data: barcode_data))
self.inventory = inventory
else
self.build_inventory
end
self.inventory.assign_attributes(inventory_attributes)
end
end
That’s it. Now, when I pass the same set of params to Work.create method, it will find the existing inventory record by barcode_data and update it.
Today you learned
Almost every internal, built-in feature on Rails can be overriden. And, yes, don’t do this unless you really need it, usually there is a way to achieve the desired behavior without overriding. But, if you need to hack – do it, but remember to test it properly.
