Running the following aggregation pipeline query against the air_alliances and air_airlines datasets from the M121 course, the sample documents returned do not display the contents of the returned as: "alliance" array (you can see the empty array in the attached screenshot).   The subsequent match query displays the results correctly, so this is a display bug not an aggregation pipeline bug.

 db.air_airlines.aggregate(
[
{"$match": {"country":

}},
{"$lookup": {
"from": "air_alliances",
"foreignField": "airlines",
"localField": "name",
"as": "alliance"
}},
{"$match": {"alliance.name": "OneWorld"}}
])