mastodon/app/controllers/api/v1/follows_controller.rb

class Api::V1::FollowsController < ApiController
  before_action -> { doorkeeper_authorize! :follow }
  respond_to    :json

  def create
    raise ActiveRecord::RecordNotFound if params[:uri].blank?

    @account = FollowService.new.call(current_user.account, target_uri).try(:target_account)
    render action: :show
  end

  private

  def target_uri
    params[:uri].strip.gsub(/\A@/, '')
  end
end