rails5: generic grid - how to get column names for current search - ruby-on-rails

Question

I wan to do a generic grid, that could be used in my app to show data from different Models. As I am a newbie in RoR - please let me know if my "vision" is ok, or i should change the way i think about this problem:
Controller:
class Admin::UsersController < ApplicationController
before_action :authenticate_user!
authorize_resource
def index
#current_scope = params[:role]
#users = User.select(:id, :email, :role, :last_sign_in_at).role(#current_scope)
end
end
User Model:
scope :role, -> (role) {where('role = ?', role) if role.present?}
Index view:
<%= render '/layouts/shared/grid', object: #users%>
Grid partial:
<% object.column_names.each do |column_name| %>
<th><%= column_name %></th>
<% end %>
My problem is that i can show always ALL columns (that is what ActiveRecord::Relation returns), not only columns for parameters selected in my query. I see few potential solutions - which one would be the best?
usage of ActiveRecord::Base.connection.exec_query - if i do so, i could use then .columns method that returns what i need. But i am not sure if i like it...
usage of search_params instead of query result to iterate
Any other suggestion?

Answer 1

You can use values method. It's not very documented but it returns parsed sql query.
For this query:
User.select(:id, :email, :role,:last_sign_in_at).role(#current_scope).values
It returns something like this:
{:select=>[:id, :email, :role, :last_sign_in_at], :where=>#<ActiveRecord::Relation::WhereClause:0x007fe0d3b29bf0 #predicates=["role = admin"], #binds=[]>}
If a query will not use select when values will not return select result too. In this case, you need to use column_names.
Too I can recommend you wrap table code to decorator. This is raw example:
class TableDecorator
def initialize(scope)
#scope = scope
end
def header_columns
scope.values[:select] || #scope.column_names
end
def columns
#scope
end
end
# controller
#table = TableDecorator.new(#users)
# index view
<%= render '/layouts/shared/grid', locals: {table: #table} %>
# grid partial
<table>
<thead>
<tr>
<% table.header_columns.each do |column_name| %>
<th><%= column_name %></th>
<% end %>
</tr>
</thead>
<tbody>
<% table.columns.each do |item| %>
<tr>
<% table.header_columns.each do |column_name| %>
<td><%= item[column_name] %></td>
<% end %>
</tr>
<% end %>
</tbody>
</table>