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# config/initializers/doorkeeper.rb

Doorkeeper.configure do
  # ...

  resource_owner_authenticator do
    user = # ... the_code_that_finds_user, varies from app to app, warden.user works if you do Devise
    if user
      user
    else
      redirect_to(login_or_register_path(after_login_path: request.fullpath)
    end
  end
end

# ----

# in login controller
def login
  user = the_code_to_login_user # varies from app to app

  if user
    login(user)

    if params[:after_login_path] # this you need to make sure to pass from links to initial redirect path
      redirect_to(params[:after_login_path])
    end
  else
    # handle bad username and password
  end
end

# in register user controller
def create
  user = User.new(permitted_user_params)
  if user.save
    login(user)

    if params[:after_login_path] # this you need to make sure to pass from links to initial redirect path
      redirect_to(params[:after_login_path])
    end
  else
    # handle invalid user
  end
end

Check out for additional info: https://github.com/doorkeeper-gem/doorkeeper/issues/955
You may also want to return token in SPA application https://github.com/doorkeeper-gem/doorkeeper/issues/846