Aggregate $match _id $eq $toObjectId not working

Using Compass 1.30.1, I was testing an aggregation and getting unexpected results. A $match was not working as expected. The simplified aggregation is:

db.getCollection('dlsComponents').aggregate([
  { $match: { library: 'Library1', collection: 'Collection1', media: 'Images', object: 'Image3' } }
 ])

And this gives the expected result by finding a document:

{ _id: ObjectId("61fc458b46d7874a3a97ef79"),
  library: 'Library1',
  collection: 'Collection1',
  media: 'Images',
  object: 'Image3',
  info: 'Image: 1/1/Images/Image3 Info', …

try to get the same document by _id:

db.getCollection('dlsComponents').aggregate([
  { $match: { _id: { $eq: { $toObjectId: "61fc458b46d7874a3a97ef79" } } } }
 ])

does not find a document. Why does the second $match not find a document?

I found that:

{ $match: { $expr: { $eq: [ ‘$_id’, ‘$$imageId’ ] } } }

does work ($$imageId is an ObjectId used in the non-simplified aggregate). Maybe the { $eq: ‘$value’ } format does not work in pipelines.

1 Like

Hi David,

The $eq used in find()/$match (without $expr) must specify an exact value: https://docs.mongodb.com/manual/reference/operator/query/eq/

You can use $eq: ObjectId("...")

Jess

1 Like

Your last post made me think that may be $toObjectId works only inside $expr. I tried

{ $match: { $expr : { $eq: [ '$_id' , { $toObjectId: "61fc458b46d7874a3a97ef79" } ] } } }

and it works.

4 Likes

See on a related topic

and

even gpt4 doesn’t solve my problem, thank

Did you told GPT4 what was your problem? Or did you do like you did here., just saying that you have a problem.