ruby on rails - How does format.html know what to render? -

i have following piece of code in file app/mailers/user_mailer.rb . how format.html know render . not in action of controller. understand basic fact renders show view default here no controller no action. how still work ?

class usermailer < actionmailer::base   def signup_notification(user)     @recipients = "#{user.email}"     @from = "#{sender.email}"     mailer_name = "signup_notification"     mail(:to => @recipients, :from => @from) |format|       format.html     end   end 

you're right actionmailer not controller (that's actioncontroller). respond_to method of actioncontroller , mail method of actionmailer entirely different things, both have concept of "what kind of response should sending" , rails devs chose offer similar interface format picking in both.

in actioncontroller respond_to method takes block , uses http request environment pick format render.

in actionmailer, mail takes block , uses different logic decide format render. when give format.html looks in views folder templates <method_name>.<format>.<acceptable_template_type>, e.g., signup_notification.html.erb. aside, should name methods in snake_case (signup_notification). uppercase method names non-idiomatic , might cause problems.

technical explanation

while don't need know use actionmailer, think it's interesting walk through flow of how template generation works here. includes interesting meta-programming logic simpler in regular controllers (no http environment).

you can see in source mail calls a function render messages. if gave mail block, mail creates instance of actionmailer::collector (which includes abstractcontroller::collector) , yields collector. so, in example collector gets function html called on (since do |format| ... end has argument, format collector instance when yield(collector) called here.

so instance of actionmailer::collector passed do |format| ... block, has function html called on it, collector uses method missing tricks abstractcontroller::collector can respond mime type, passing requested type on actionmailler::collector#custom builds email looking in views folder templates matching type.