Diffing two collections

jakey___last year

6

• 

  dribbens

  last year

  You're already ahead of my knowledge it seems with using aggregations for this. It is essentially just comparing JSON for differences?

  
  

  My approach might be to write out the diff as the changes are written instead of trying to compare them in a read operation.

• 

  jakey___last year

  Oh, thanks for responding @dribbens ! I forgot I posted this. I did end up getting an aggregate going that allowed me to tell the docs that changed between two collections.

  
  
  

  [
      {
          $lookup: { // provides the mutable data to compare against
              from: "immutable-data-collection-name",
              localField: "_id",
              foreignField: "_id",
              as: "mutableData",
          },
      },
      {
          $addFields: { // Filters the data down to only that which has changed and puts it in a new field called "diff"
              diff: {
                  $filter: {
                      input: "$mutableData",
                      cond: {
                          $or: [
                              { $ne: ["$$this.Name", "$Name"] },
                              // ...and so on for all the other relevant fields you want to check against when determining an change has happened.
                          ],
                      },
                  },
              },
          },
      },
      {
          $match: { // further filters out any documents that have no changes, leaving only the ones that have changed
              $expr: { $gt: [ { $size: "$diff", }, 0, ], },
          },
      },
  ]

  
  

  
  

  Then to actually use this in a payload endpoint, I ran code that looked like the following:

  
  
  

  const getDiff = () => {
      return new Promise((resolve, reject) => {
          payload.collections['mutable-data-collection-name'].Model.aggregate(diffAggregate)
          .then((result) => {
              resolve(result)
          })
          .catch((err) => {
              reject(err)
          })
      })
  }

  
  

  
  

  diffAggregate

  being the initial array of objects code I pasted.

  
  

  
  

  Props to chatGPT for helping me understand how this could work, lol. Also to @jmikrut for responding with how to access the mongoose model in this channel

  https://discord.com/channels/967097582721572934/1041606968755896370

  . Very helpful!

• 

  dribbens

  last year

  Marking as Answered so it shows up in our site's community help. Great work @jakey___!