Rails APIでcamelCase ref: https://qiita.com/vochicong/items/d64f3b3d5a448a3b1f42

file0.rb

# File: config/initializers/params_snakeizer.rb
# Transform JSON request param keys from JSON-conventional camelCase to
# Rails-conventional snake_case
module ActionController
  # Modified from action_controller/metal/strong_parameters.rb
  class Parameters
    def deep_snakeize!
      @parameters.deep_transform_keys!(&:underscore)
      self
    end
  end
end

file0.txt

# File: config/initializers/params_snakeizer.rb
# Transform JSON request param keys from JSON-conventional camelCase to
# Rails-conventional snake_case
module ActionController
  # Modified from action_controller/metal/strong_parameters.rb
  class Parameters
    def deep_snakeize!
      @parameters.deep_transform_keys!(&:underscore)
      self
    end
  end
end

file1.rb

# File: app/controllers/application_controller.rb
class ApplicationController < ActionController::API
  protected

    # Snakeize JSON API request params
    def snakeize_params
      params.deep_snakeize!
    end
end

file1.txt

# File: app/controllers/application_controller.rb
class ApplicationController < ActionController::API
  protected

    # Snakeize JSON API request params
    def snakeize_params
      params.deep_snakeize!
    end
end

file2.rb

class UsersController < ApplicationController
  before_action :snakeize_params, only: [:say_hello, :say_goodbye]
  # .....
end

file2.txt

class UsersController < ApplicationController
  before_action :snakeize_params, only: [:say_hello, :say_goodbye]
  # .....
end

file3.rb

# File: app/controllers/application_controller.rb
class ApplicationController < ActionController::API
  before_action :snakeize_params

  protected

    # Snakeize JSON API request params
    def snakeize_params
      params.deep_snakeize!
    end
end

file3.txt

# File: app/controllers/application_controller.rb
class ApplicationController < ActionController::API
  before_action :snakeize_params

  protected

    # Snakeize JSON API request params
    def snakeize_params
      params.deep_snakeize!
    end
end

file4.rb

json.key_format! camelize: :lower

file4.txt

json.key_format! camelize: :lower

file5.rb

Jbuilder.key_format camelize: :lower

file5.txt

Jbuilder.key_format camelize: :lower