Chapter 3 - Lab $lookup - What's wrong with pipeline?

MongoDB University M121: The MongoDB Aggregation Framework
#1

I am using the following aggregation pipeline which is matching to what is mentioned in the answer:

var pipeline1 = [
{ $match: { airplane: /747|380/ } },
{ $lookup: { from: “air_alliances”, localField: “airlines”, foreignField: “airline.name”, as: “alliance”} },
{ $unwind: “$alliance” },
{ $group: { _id: “$alliance.name”, count: { “$sum”: 1 } } },
{ $sort: { count: -1 } }
]

But executing this pipeline is giving me wrong result.

Can someone let me know what’s wrong with this pipeline?

#2

Help us help you by telling us more about what is wrong with the result you get. Wrong order? Wrong list of documents?

#3

Are you sure about:

See $lookup (aggregation) — MongoDB Manual for the syntax and the significance of localField and foreignField. There is no field named airlines in the input documents from air_routes.

#4

I am executing this pipeline against air_routes. The localField “airlines” is from dataset air_alliance. The pipeline produced the below result:

{ “_id” : “SkyTeam”, “count” : 89}
{ “_id” : “OneWorld”, “count” : 89 }
{ “_id” : “Star Alliance”, “count” : 89 }

#5

That’s correct. So your input documents are from air_routes.

The above is not correct. That is why I mentioned

and

This is also why I mentioned

From the detailed answer of the lab they have:

    foreignField: "airlines",
    localField: "airline.name",

but you have

    localField: "airlines",
    foreignField: "airline.name"
1 Like
#6

Hey @Naveen_Mundhra,

I see that you already completed the Lab: Using $lookup :clap:

However, if you still have any additional questions, please feel free to reach out.

Kind Regards,
Sonali