CODEWITHSUNDEEP
ruby-on-railsmongodbruby-on-rails-4mongoid
Harsha M V
Harsha M V

Reputation: 54949

Calculating the Count of a related Collection

I have two models Professionals and Projects

Professionals hasMany Projects

Projects belongsTo Professionals

In the Professionals index page i need to show the number of projects the Professional has.

Right now i am doing the following query to get all the Professionals. How can i fetch the count of the Projects of each of the Professionals as well.

@pros = Professionals.all.asc(:name)

Upvotes: 3

Views: 84

Answers (2)

Neil Lunn
Neil Lunn

Reputation: 151132

I am just abstracting here because the rails thing really isn't my bag. But let's talk about schema and things to look for. And as such the code is really just "pseudo-code" but should be close to what is wanted.

Considering "just" how MongoDB is going to store the data, and that you presumably seem to have multiple collections. And I am not saying that is or is not the best model, but just dealing with it.

Let us assume we have this data for "Projects"

{
    "_id" : ObjectId("53202e1d78166396592cf805"),
    "name": "Project1,
    "desc": "Building Project"
},
{
    "_id" : ObjectId("532197fb423c37c0edbd4a52")
    "name": "Project2",
    "desc": "Renovation Project"
}

And that for "Professionals" we might have something like this:

{
    "_id" : ObjectId("531e22b7ba53b9dd07756bc8"),
    "name": "Steve",
    "projects": [
        ObjectId("53202e1d78166396592cf805"),
        ObjectId("532197fb423c37c0edbd4a52")
    ]
}

Right. So now we see that the "Professional" has to have some kind of concept that there are related items in another collection and what those related items are.

Now I presume, (and it's not my bag) that there is a way to get down to the lower level of the driver implementation in Mongoid ( I believe that is Moped off the top of my head ) and that it likely ( from memory ) is invoked in a similar way to ( asssuming "Professionals" as the class model name ) :

Professionals.collection.aggregate([
    { "$unwind": "$projects" },
    { "$group": {
        "_id": "$_id",
        "count": { "$sum": 1 }
    }
])

Or in some similar form that is more or less the analog to what you would do in the native mongodb shell. The point being, with something like this you just made the server do the work, rather than pulling all the results to you client and looping through them.

Suggesting that you use native code to iterate results from your data store is counter productive and counter intuitive do using any kind of back end database store. Whether it by a SQL database or a NoSQL database, the general preference is as long as the database has methods to do the aggregation work, then use them.

If you are writing code that essentially pulls every record from your store and then cycles through to get the result then you are doing something wrong.

Use the database methods. Otherwise you might as well just use a text file and be done with it.

Upvotes: 2

j-dexx
j-dexx

Reputation: 10416

I would add projects_count to Professional

Then

class Project
  belongs_to :professional, counter_cache: true
end

And rails will handle the count every time a project is added to or removed from a professional. Then you can just do .projects_count on each professional.

Edit: If you actually want additonal data 

@pros = Professionals.includes(:projects).order(:name)

Then

@pros.each do |pro|
  pro.name
  pro.projects.each do |project|
    project.name
  end
end

Upvotes: 2

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