I created new Rails App and install Devise and omniauth-facebook gem.
And setting my Facebook App, as testing environment.
So, I logged in via facebook and signe
I found the way to get email address myself.
https://developers.facebook.com/docs/apps/changelog#v2_4
This doc says that
Declarative Fields To try to improve performance on mobile networks, Nodes and Edges in v2.4 requires that you explicitly request the field(s) you need for your GET requests. For example, GET /v2.4/me/feed no longer includes likes and comments by default, but GET /v2.4/me/feed?fields=comments,likes will return the data. For more details see the docs on how to request specific fields.
so, it need to write in config/initializers/devise.rb
config.omniauth :facebook, 'app_id', 'app_secret', scope: 'email', info_fields: 'email'
scope: 'email'
is defaultnot only scope, but also info_fields.