Rails 4 - sum values grouped by external key - ruby-on-rails

Question

I know this must be simple but I'm really lost.
Three models: Job, Task and Operation, as follows:
Job
has_many :tasks
Task
belongs_to :job
belongs_to :operation
Operation
has_many :jobs
Job has an attribute, total_pieces, which tells me how many pieces you need.
For each Job, you can add a number of Tasks, which can belong to different Operations (cutting, drilling, etc.) and for every task you can set up a number of pieces.
I don't know in advance how many Operations will be needed for a single Job, but I need to alert user of the number of pieces left for that Operation when a new Task is inserted.
Let us make an example:
Job 1: total_pieces=100
- Task 1: operation 1(cutting), pieces=20
- Task 2: operation 1(cutting), pieces=30
- Task 3: operation 2(drilling), pieces=20
I need to alert the user that they still need to cut 50 pieces and to drill 80.
Hypothetically, if i add:
- Task 4: operation 3(bending), pieces=20
I need to alert the user that they also still need to bend 80 pieces.
So far i've managed to list all kinds of Operations for each Job using map, but now i need to sum up all pieces of the Task with the same Operation type in a Job, and only for those Operations present in the Tasks belonging to that Job.
Is there any way to do this using map? Or do I need to write a query manually?
EDIT: this is what I managed to patch up at the moment.
A method operations_applied in Job gives me a list of ids for all the Operations usend in Tasks queued for the Job.
Then another method, pieces_remaining for(operation) gives me the remaining pieces for the single operation.
Finally, in the Job views I need, I iterate through all operations_applied printing all pieces_remaining_for.
I know this is not particularly elegant but so far it works, any ideas to improve this?
Thank you.

Answer 1

If I'm not misunderstanding, it is not possible to do what you want to do with map, since map always applies to arr.size == arr.map {...}.size and you want to reduce your array.
What you could do is something like this:
job = Jobs.first
operation_pieces = {}
job.tasks.each do |task|
operation_pieces[task.operation.id] ||= { operation: task.operation }
operation_pieces[task.operation.id][:pieces] ||= 0
operation_pieces[task.operation.id][:pieces] += task.pieces
end
Now operation_pieces contains the sum of pieces for the operation with the id of the respective index. But I'm sure there is a more elegant version to do this ;)
EDIT: changed the code example to a hash
EDIT: and here is the more elegant version:
job = Jobs.first
job.tasks
.group_by(&:operation)
.map { |op, tasks|
{ op => tasks.sum(&:pieces) }
}
The group_by groups your array of tasks by the operation of the task (maybe you need to use group_by { |t| t.operation } instead, I'm not sure) and inside the map afterwards the pieces of each task with the same operation is summed up. Finally, you end up with a hash of the type OPERATION => PIECES_SUM (INTEGER).

Answer 2

I assume the following variables required for your query,
attributes
Task:
number_of_pieces,
Job:
name,
Operation:
name
Job.joins("LEFT JOIN tasks ON jobs.id = tasks.job_id").joins("LEFT JOIN operations ON operations.id = tasks.operation_id").select(" SUM(tasks.number_of_pieces) as number_of_pieces, operations.name, jobs.name").group("operations.id, jobs.id")
This will list all the jobs, and sum of pieces required for each operation under it.
if you have the job_id for you want the list of operations and the pieces, then use the below code,
Job.find(params[:job_id]).joins("LEFT JOIN tasks ON jobs.id = tasks.job_id").joins("LEFT JOIN operations ON operations.id = tasks.operation_id").select(" SUM(tasks.number_of_pieces) as number_of_pieces, operations.name, jobs.name").group("operations.id, jobs.id")
please put in comments if your need any explanation.