$graphLookup and FieldPath field names may not start with a $

MongoDB University M121: The MongoDB Aggregation Framework
#1

Good afternoon,

Q1. So, I had a little time so I thought I would ask, why in $graphLookup the connectFromField and ToField, are meant to be referenced without preceeding a $

$graphLookup: {
      from: 'child_reference',
      startWith: '$direct_reports',
      connectFromField: 'direct_reports',
      connectToField: 'name',
      as: 'upto_level_2_reports',
      maxDepth: 1,
     depthField: 'level'
}

Obviously my understanding is limited, but in other stages we reference pre existing fields with a $, apart from the match stage, unless using the $expr.

I just wondered what bit of logic I was missing, or is this one of those “Exceptions”?

Q2. So the output from the $graphLookup, via the as: seems a little random, so I assume that the normal process would be to $unwind the results, and use the depth field to put a bit of structure to the documents?

Is there a way to actually get the output in the such that children documents were within the parent document?

#2

If you look at the documentation at

you will see that startWith accept an expression, so the need of $ to indicate that it is a field name.

Any reason for the the double star signs at the end of each lines in your $graphLookup?

2 Likes
#3

@steevej-1495

I forgot to edit them out, when I was bolding/unbolding.

Thank you for that, since it just gave me a little light bulb moment, about when to use $fieldName and when not to, instead of it being a bit of a suck it and see strategy or copy a piece of code that works :wink:

At least I will have more of an idea, when to use $ and what expression really means.