Question

I want to store “carpool_debts” which is basically going to hold the number of days owed to other users. It looks like this:

carpool_debts{
              _id,
              owner,
              owner_id,
              creditors:[{name,
                          id,
                          amount},
                          {name,
                           id,
                           amount}
                        ]} 

Does that data structure look reasonable for what I want to store? Also implementing that data structure seemed cumbersome to maintain. I found it cumbersome mainly because there isn’t an upsert type of function available in meteor yet. Instead of creditors being a list of sub documents would I be better off storing the creditors as a delimited string? I would like to know if I am on the right path or if I am missing something? Thanks.

Answer 1

You can structure mongo documents just like you would in a relational database, for example, having separate collections for creditors and owners and using carpool_debts as a link table with the amount attached:

carpool_debts{
              _id,
              owner_id,
              creditor_id,
              amount}

creditors{
          _id,
          name}

owners{
       _id,
       name}

However, this is not using mongodb to its full potential. Especially if this is a database with masses of data, you may want to optimise it for the most used queries, otherwise it’ll be slow. For example, to optimise for looking up an owner’s debt, you can add the data needed right there in the owners collection, using sub documents for creditors, and sub documents again for individual debts, similar to what you’ve already done:

owners{
       _id,
       name,
       creditors: {id,
                   name,
                   debts: {
                           amount,
                           due_date}
                  }
      }

and similarly, add the debt information on the creditors collection if you often look up the outstanding debt of creditors:

creditors{
          _id,
          name,
          debtors: {
                    owner_id,
                    owner_name, 
                    debts: { 
                             aount,
                             due_date
                           }
                   }
         }

This way, you only need to look up one record to get all the information you need. Of course, there are catches. First of all, this is not very DRY, but that’s intentional. But you have to remember to update the other table(s) when something changes. If you change the name of a creditor for example, you’ll need to update every owner document that has debts with this creditor (make sure you index that). This of course makes updates much slower (and the database bigger), but if you don’t update very often, and look up much more often, this is not going to be a problem.
Also if for example creditors can have thousands of individual outstanding debts, you may have to separate that into a link table, or rather, link collection, like this, so you don’t exceed mongodb’s maximum document size:

creditors{
          _id,
          name,
         }

debtors: {
           owner_id,
           creditor_id, 
           debts: { 
                    amount,
                    due_date
                  }
          }

Then you have one document for each creditor-owner connection. This means more documents to look up when looking at a creditor, but still just one for looking up an owner.