rails 4 how to use where and where in condition simultaneously

You could use arel, but I'd just do something like:

@posts = Post.where("category_id = ? AND product_model_id IN (?)", id, model)

model_ids = model.split(",").map(&:to_i)
@posts = Post.where(category_id: id, product_model_id: model_ids)

or

model_ids = model.split(",").map(&:to_i)
@posts = Post.where("category_id = ? AND product_model_id IN (?)", id, model_ids)

According to the rails guide, you can pass in an array to where and it should understand that you want to use IN. See the guide here: http://guides.rubyonrails.org/active_record_querying.html#subset-conditions

I'm a little confused by your syntax, since model = (1, 2, 3, 4) doesn't look like valid array syntax.

Relevant part of the guide:

Client.where(orders_count: [1,3,5])