Question

I need a little help with Rails. I am transferring a backend system of data look up services from PHP to RoR. Which I am finding to be surprisingly concise.

I need to be able to search a model on its relation with a tag model. Each model has been given the has_and_belongs_to_many association.

When the search is passed in with a comma separated list of values I need to perform an ‘AND’ search on the tags so I can find all resources that have all tags passed (i.e. London AND bridges).

Also it needs to be case insensitive. Currently I have only a single tag search being performed in the Resource model.

def self.search(search)
    where('LOWER(tags.name) = ?', search.downcase).joins(:tags)
end

All my attempts at using an array using either where, find, or all have not been successful.

The PHP way of doing an AND search on a join table seemed rather hacky, I am hoping Rails can provide a more elegant solution.

Any help would be greatly appreciated.

Answer 1

If you need an AND, then you need to ensure that the number of matching tags is precisely the number you requested, so that involves a COUNT as well. The query you’re gunning for will involve IN.

It would be more convenient if you could ensure that your tags are always stored in lower case. Doing operations like LOWER(...) in a query is going to be trouble if you have a lot of data since it results in an automatic table scan and cannot be indexed.

Secondly, I’d avoid using has_and_belongs_to_many as this is a relic from the Rails 1.0 days that isn’t as flexible or reliable as the has_many :through alternative. The only difference between the two approaches from a database perspective is that the :through join table has a unique ID for each entry and is represented by a first-class model. The has_and_belongs_to_many system is quirky to the point of uselessness a lot of the time.

What you’re probably wanting is something that looks like this:

def self.with_all_tags(*tags)
  tags = tags.flatten.collect(&:downcase)
  where('tags.name IN (?)', tags).having('COUNT(tags.id)=?', tags.length).joins(:tags)
end

def self.with_any_tags(*tags)
  tags = tags.flatten.collect(&:downcase)
  where('tags.name IN (?)', tags).joins(:tags)
end

You may need to experiment to get the right behaviour. Your example wasn’t that specific.