how to find number of tag matches in acts as taggable on - ruby

Question

I have two entries in my database
Obj1 is tagged with "hello, world, planet"
Obj2 is tagged with "hello"
if I do modelName.tagged_with(["hello", "world", "planet", "earth"], :any=>true)
I want to sort the returned objects in order of highest to lowest number of tags matched.
so in this case i'd like the order to be Obj1, Obj2
how can I do this? is there a way to get number of tags matched for each of the returned results?

Answer 1

You can call tag_list on the objects and use that to figure out how many tags there are:
tags = %w{hello world planet earth}
objs = ModelName.taggedWith(tags, :any => true)
objs.sort_by! { |o| -(tags & o.tag_list).length }
The tags & o.tag_list yields the intersection of the tags you're looking for and the tags found, then we negate the size of the intersection to tell sort_by (which sorts in ascending order) to put larger intersections at the front, negating the result is an easy way to reverse the usual sort order.

Answer 2

Posting this here if someone else is looking for a way to query a model by tags and order by the number of matches. This solution also allows for the usage of any "equality" operator like the % from pg_trgm.
query = <<-SQL
SELECT users.*, COUNT(DISTINCT taggings.id) AS ct
FROM users
INNER JOIN taggings ON taggings.taggable_type = 'User'
AND taggings.context = 'skills'
AND taggings.taggable_id = users.id
AND taggings.tag_id IN
(SELECT tags.id FROM tags
WHERE (LOWER(tags.name) % 'ruby'
OR LOWER(tags.name) % 'java'
OR LOWER(tags.name) % 'sa-c'
OR LOWER(tags.name) % 'c--'
OR LOWER(tags.name) % 'gnu e'
OR LOWER(tags.name) % 'lite-c'
))
GROUP BY users.id
ORDER BY ct DESC;
SQL
User.find_by_sql(query)
Note that the code above will only work if you have pg_trgm enabled. You can also simply replace % with ILIKE.
EDIT: With ActiveRecord and eager loading:
This could be in a scope or class method and can be chained with other ActiveRecord methods.
ActiveRecord::Base.connection
.execute('SET pg_trgm.similarity_threshold = 0.5')
matches = skills.map do
'LOWER(tags.name) % ?'
end.join(' OR ')
select('users.*, COUNT(DISTINCT taggings.id) AS ct')
.joins(sanitize_sql_array(["INNER JOIN taggings
ON taggings.taggable_type = 'User'
AND taggings.context = 'skills'
AND taggings.taggable_id = users.id
AND taggings.tag_id IN
(SELECT tags.id FROM tags WHERE (#{matches}))", *skills]))
.group('users.id')
.order('ct DESC')
.includes(:skills)
Override skill_list from acts-as-taggable-on in the model:
def skill_list
skills.collect(&:name)
end
and proceed normally.