Are those two queries the same in rails 3.2.12?

Question

We need to retrieve logs for customer comm record in our rails app. The condition in plain english is logs for customer_comm_record based on:

#1. the sales_id in customer comm record's customers is equal to current user id
#2. the sales_id in customer comm record's customer's project is equal to the current user id. 

The SQL code for #1 could be (SQL#1):

Log.joins(:customer_comm_record =>:customer).where('customers.sales_id = ?', session[:user_id])

SQL code for #2 could be (SQL#2):

Log.joins(:customer_comm_record =>{ :customer => :projects}).where('projects.sales_id = ?', session[:user_id])

Here is what we come up (SQL#3) in one line:

Log.joins(:customer_comm_record =>{ :customer => :projects}).where('customers.sales_id = ? OR projects.sales_id = ?', session[:user_id], session[:user_id])

The SQL code returned by above in rails console is (replacing session[:user_id] with 1) :

 SELECT "logs".* FROM "logs" INNER JOIN "customer_comm_records" ON "customer_comm_records"."id" = "logs"."customer_comm_record_id" INNER JOIN "customers" ON "customers"."id" = "customer_comm_records"."customer_id" INNER JOIN "projects" ON "projects"."customer_id" = "customers"."id" WHERE (customers.sales_id = 1 OR projects.sales_id = 1)

The question is weather SQL#3 == SQL#1 UNION SQL#2. Can someone answer the question? If it is not, what's the right one? Thanks.

Answer 1

The only differences are:

• The first method will return duplicate entries where the sales_id for the customer and project both match the users id.
• The second method might not return records where a customer with a matching sales_id does not have a project (no projects.customer_id for that customer).

The last difference only matters if a customer does not necessarily have any projects. Otherwise, the second method is better because it avoids dealing with duplicates.

If a customer does not always have a project, a simple workaround would be to use includes instead of joins:

Log.includes(:customer_comm_record =>{ :customer => :projects}).
    where('customers.sales_id = :id OR projects.sales_id = :id', id: session[:user_id])

This will force a LEFT JOIN and will return customers regardless of whether they are associated with a project.