Can FactoryBot generate factories after your models have been created?

This works for me using rails g factory_bot:model User either running the command or just puts'ing the command out. You do still have to fill in the value.

@run_command        = true
@force              = true
@columns_to_ignore  = %w[id created_at update_at]
@tables_to_ignore = %w[schema_migrations ar_internal_metadata]
tables = ActiveRecord::Base.connection.tables.reject{|t| (@tables_to_ignore || []).include?(t)}

tables.each do |table|
  klass = table.singularize.camelcase.constantize
    command = "rails g factory_bot:model #{klass.to_s} #{klass.columns.reject do |c|
      (@columns_to_ignore || []).include?(c.name)
    end.map do |d|
      "#{d.name}:#{d.sql_type == 'jsonb' ? 'json' : d.type}"
  end.join(' ')}"
  command << ' --force' if @force
  puts command
  puts %x{#{command}} if @run_command
  puts (1..200).to_a.map{}.join('-')
end

Configure Factory Bot as the fixture replacement so you do not have to create factories manually.

In config/application.rb:

config.generators do |g|
  g.test_framework :rspec, fixture: true
  g.fixture_replacement :factory_bot, dir: 'spec/factories'
end

For more: https://github.com/thoughtbot/factory_bot_rails/blob/master/features/fixture_replacement_config.feature

Not exactly related.

I also built a gem to build factory from existing data.

Hopefully, it can help you speed up the process a bit......

puts FactoryBotFactory.build(User.new, file_path: 'spec/factories/user.rb')
puts FactoryBotFactory.build(User.last, file_path: 'spec/factories/user.rb')

# example output from User.new
FactoryBot.define do
  factory :user, class: User do
    id { nil }
    name { nil }
    created_at { nil }
    updated_at { nil }
    display_name { nil }
    image_url { nil }
    is_active { true }
  end
end

You can also configure customize converter if you need to built fake data.