seems that is possible to create complex joins with no associations between models
first i have tried to use the table name in the :joins/:include parameters area (see the Eager loading of associations in Active Record doc)
conditions[:joins] = "comments"
but doesn't work if there is no association between the model and even if is trough another table/model it doesn't work like i expected
so the solution is to add another join expression in the conditions arguments
conditions[:joins] = "LEFT JOIN comments ON comments.post_id = id"
and then you can filter on comments content or id
then filter the model and use the results