Rails: AND operator in a has_many association - ruby-on-rails

Question

My relationship is a Client can have many ClientJobs. I want to be able to find clients that perform both Job a and Job b. I'm using 3 select boxes so I can pick a maximum of three jobs to select from. The select boxes are populated from the database.
I know how to test for 1 job with the query below. But I need a way to use an AND operator to test that both jobs exist for that client.
#clients = Client.includes("client_jobs").where(
client_jobs: { job_name: params[:job1]})
Unfortunately it's easy to do an IN operation like below, but I'm thinking the syntax for AND should be similar....I hope
#lients = Client.includes("client_jobs").where(
client_jobs: { job_name: [params[:job1], params[:job2]]})
EDIT: Posting the sql statement that hits the database from the answer below
Core Load (0.6ms) SELECT `clients`.* FROM `clients`
CoreStatistic Load (1.9ms) SELECT `client_jobs`.* FROM `client_jobs`
WHERE `client_jobs `.`client_id` IN (1, 2, 3, 4, 5, 6, 7, 8, 9, 10,........)
The second query runs through every client_job in the database. It's never tested against the params[:job1], params[:job2] etc. So #clients returns nil crashing my view template
(undefined method `map' for nil:NilClass

Answer 1

In my opinion, a better approach then self-joins is to simply join ClientJobs and then use GROUP BY and HAVING clauses to filter out only those records that exactly match the given associated records.
performed_jobs = %w(job job2 job3)
Client.joins(:client_jobs).
where(client_jobs: { job_name: performed_jobs }).
group("clients.id").
having("count(*) = #{performed_jobs.count}")
Let's walk through this query:
first two clauses join the ClientJobs to Clients and filter out only those, that have any of the three jobs defined (it uses the IN clause)
next, we group these joined records by Client.id so that we get the clients back
finally, the having clause ensures we only return those clients that had exactly 3 ClientJob records joined in, i.e. only those that had all the three client jobs defined.
It is the trick with HAVING(COUNT(*) = ...) that turns the IN clause (which is essentially an OR-ed list of options) into a "must have all these" clause.

Answer 2

To do this in a single SQL query try the following:
jobs_with_same_user = ClientJob.select(:user_id).where(job_name: "<job_name1>", user_id: ClientJob.select(:user_id).where(job_name: "<job_name2>"))
#clients = Client.where(id: jobs_with_same_user)
Here's what this query is doing:
Select the user_ids of all Client jobs with [job_name2]
Select the user_ids of all Client jobs with user_id IN result set from (1) AND having [job_name1]
Select all users with using (2) as a subquery.
Not many know this but Rails 4+ supports subqueries. Basically this is a self join acting as subquery for the clients:
SELECT *
FROM clients
WHERE id IN <jobs_with_same_user>
Also, I'm not sure if you're referencing the client_jobs association in your view, but if you are, add the includes statement to avoid an N+1 query:
#clients = Client.includes(:client_jobs).where(id: jobs_with_same_user)
EDIT
If you prefer, the same result can be achieved with a self-referencing inner join:
jobs_with_same_user = ClientJob
.select("client_jobs.user_id AS user_id")
.joins("JOIN client_jobs inner_client_jobs ON inner_client_jobs.user_id=client_jobs.user_id")
.where(client_jobs: { job_name: "<first_job_name1>" }, inner_client_jobs: { job_name: "<job_name2>" })
#clients = Client.where(id: jobs_with_same_user)