[RUBY] How to automatically generate ER diagram when migrating with Rails6


We have summarized how to automatically generate ER diagram in Rails6. In the case of Rails6, Zeitwerk is used by default for autoloading files, but due to this, ER diagram could not be generated well, so the workaround is also described.

Development environment

Rails: 6.0.3 Ruby:2.7.1 Macbook Pro Graphviz GraphQL (This was due to the automatically generated file.)

ER diagram settings

Install Gem.


group :development do
  gem 'rails-erd'

Set to gitignore if necessary.



You can generate an ER diagram with the following command.

$ bundle exec erd

Auto-generated settings

To automatically generate ER diagram at the time of migration, set in Rakefile. If you don't need automatic generation, you don't need to mention it here.


#Hook the task of migrate
Rake::Task['db:migrate'].enhance do
  if Rails.env.development?

#Tasks after migrate
task after_migrate: :environment do

#Create ER diagram
task create_erd: :environment do
  # attributes=foreign_keys,primary_keys,timestamps (Attributes display primary key, foreign key, timestamp)
  # sort=false (Do not sort column names alphabetically)
  # filename=loof-api-server-erd (file name)
  # filetype=dot (File extension)
  sh 'bundle exec erd --attributes=foreign_keys,primary_keys,content,timestamps --sort=false --filename=hogehoge --filetype=dot'

Error countermeasures

Not recommended measures

When I stopped using zeitwerk, which autoloads files, and changed it to classic, I was able to automatically generate it, but Rspec did not pass.


config.autoloader = :classic

About zeitwerk uninitialized constant XXX (NameError) error [Rails Guide 2 Enable Zeitwerk Mode](https://railsguides.jp/autoloading_and_reloading_constants.html#zeitwerk%E3%83%A2%E3%83%BC%E3%83%89%E3%82%92% E6% 9C% 89% E5% 8A% B9% E3% 81% AB% E3% 81% 99% E3% 82% 8B) [When Rails Guide 11 Zeitwerk is not used](https://railsguides.jp/autoloading_and_reloading_constants.html#zeitwerk%E3%82%92%E4%BD%BF%E3%82%8F%E3%81%AA%E3 % 81% 84% E5% A0% B4% E5% 90% 88)

Measures taken

When I generated the ER diagram, I got the following error.


$ bundle exec erd
Failed: Zeitwerk::NameError: expected file /app/graphql/interface_types/base_interface.rb to define constant Types::BaseInterface, but didn't

Apparently the module definition method is not appropriate. How to break Zeitwerk

The cause was the file automatically generated by GraphQL, so I changed it as follows.


module Types
  module BaseInterface
    include GraphQL::Schema::Interface
    field_class Types::BaseField 

After measures ↓


module Types::BaseInterface
  include GraphQL::Schema::Interface
  field_class Types::BaseField

Change other files as well.


module Types
  class BaseScalar < GraphQL::Schema::Scalar

After measures ↓


class Types::BaseScalar < GraphQL::Schema::Scalar

When I generated the ER diagram again, the following error appeared this time.


$ bundle exec erd
Failed: RuntimeError: Saving diagram failed!
Verify that Graphviz is installed and in your path, or use filetype=dot.

It is necessary to specify file as dot.

Terminal(Specify file)

$ bundle exec erd --filetype=dot

You have successfully generated an ER diagram!

At the end

Since Rails6 x GraphQL was used, the auroload could not be performed correctly, and the error occurred.

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