ransack_syntax.rb

# search(column name ('or' to be able to search in several columns + type de recherche, e.g. 'end' - last characters of the column, 'cont' - contains)
User.search('email_end' => 'gmail.com').result
User.search('first_name_or_last_name_cont' => 'bob').result

ActiveAdmin.describe User do
  filter :first_name_cont, label: 'First Name'
end

# association name + name of column in the associated model + type of search
ActiveAdmin.describe User do
  filter :preferences_content_cont, label: 'Preferences'
end