FactoryBot .find_or_create_by

Joey Cheng — Mon, 18 Mar 2019 05:59:58 +0000

TL;DR

factory :country do
  to_create do |instance|
    instance.id = Country.find_or_create_by(name: instance.name, code: instance.code).id
    instance.reload
  end
end

How did I come up with that solution? See below.

There seems to be no clear documentation on how to go about this. So, here's what I tried.

Before we begin, let's prepare our test environment, we will be using a Country model, with attributes name and code:

# models/country.rb

class Country
  validates :name, presence: true
  validates :code, presence: true, length: { is: 2 }, uniqueness: { case_sensitive: false } # ISO-3166-ALPHA-2
end

# spec/factories/countries.rb

FactoryBot.define do
  factory :country do
    name { 'Canada' }
    code { 'CA' }
  end
end

Now, we're all good engineers and good engineers write unit tests. This will make verifying results easier. The RSpec.

Attempt 1. Overriding the `initialize_with` block

This is the most voted answer on a question posted on StackOverflow. We can override a factory's default initialization behavior with:

factory :country do
  initialize_with { Country.find_or_create_by(name: name, code: code) }
end

While the solution does work, it breaks the factory. Calling build(:country) will now create a record in database if it does not exist.

Looking at the comments section, we see a suggestion to use .find_or_initialize_by:

factory :country do
  initialize_with { Country.find_or_initialize_by(name: name, code: code) }
end

This partially restores behaviour of build as it does not create a record anymore. However, it still returns a persisted record if found.

This approach breaks the original behaviour of build, which is to only initialize a record, and not return a persisted record. There is also a performance cost of accessing the database to perform a Country.find_by. An ideal solution would be to only access the database on create, to create the record. We should not need to read or write to database on build.

RSpec Results

Attempt 2. Overriding the `to_create` block (v1)

Digging into FactoryBot documentation, we see a section on Custom Methods to Persist Objects.

By default, creating a record will call save! on the instance; since this may not always be ideal, you can override that behavior by defining to_create on the factory

Let's try it out:

factory :country do
  to_create { |instance| Country.find_or_create_by(name: instance.name, code: instance.code) }
end

RSpec Results

From the results, we see that build and create works as expected.
However, create seems to return the factory-pre-save instance instead of the created instance, which explains why the country.id is nil, but Country.count increased. 🤔

Alright, we're close. All we need is a way to set the attributes back to it's own instance after calling the .find_or_create_by so it can assume identity of the found-or-created object.

Attempt 3. Overriding the `to_create` block (v2)

Now, we know the instance in to_create { |instance| ... } is the model object itself, which is an ActiveRecord model. Hence, we should be able to sort-of-force-update-the-attributes like so:

factory :country do
  to_create { |instance| instance.attributes = Country.find_or_create_by(name: instance.name, code: instance.code).attributes }
end

RSpec Results

Oh 💩it worked. But one last hurdle - country.persisted? returns false. Easiest way to fix this is by .reload-ing the instance:

factory :country do
  to_create do |instance|
    instance.attributes = Country.find_or_create_by(name: instance.name, code: instance.code).attributes
    instance.reload
  end
end

Yay, all green! 🚦

RSpec Results

But, #reload is not the ideal solution, as it performs an unnecessary database read, when all we want is for #persisted? to behave correctly. Looking at the Rails documentation, we find that #persisted? is the inverse of #new_record?, which takes its value from instance variable @new_record.

Which means, we could possibly do this:

factory :country do
  to_create do |instance|
    instance.attributes = Country.find_or_create_by(name: instance.name, code: instance.code).attributes
    instance.instance_variable_set('@new_record', false)
  end
end

Damn it worked 😎

RSpec Results

Now, I agree this is hacky. Probably should just stick to .reload to be safe. If this is not relevant to you, probably could just ignore the .reload altogether.

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DEV Community: Joey Cheng

FactoryBot .find_or_create_by

TL;DR

Attempt 1. Overriding the initialize_with block

Attempt 2. Overriding the to_create block (v1)

Attempt 3. Overriding the to_create block (v2)

Attempt 1. Overriding the `initialize_with` block

Attempt 2. Overriding the `to_create` block (v1)

Attempt 3. Overriding the `to_create` block (v2)