qc-additions

This gem adds some methods to queue_classic queues that I found helpful:

• enqueue_if_not_queued(method, *args)
• job_count(method, *args)
• job_exists?(method, *args)

An index might help to speed up the job_count and job_exists? queries. Although, I haven't really tested if there is much to gain by adding this:

CREATE INDEX idx_qc_unlocked_job_count ON queue_classic_jobs (q_name, method, args) WHERE locked_at IS NULL;

The method arguments are serialized to JSON. However, the comparison performed when looking for jobs in the database is a string comparison. Results might be incorrect if there is more than one way to serialize the arguments to a JSON string. It should be safe for simple things like passing a numeric id.

I have some of the original code from there:

• QueueClassic/queue_classic#92

• https://github.com/GreenplumChorus/queue_classic/commit/2719301c2813717692169c1eeab42d317df0ac59