M001: Chapter 4 Lab 1

Working with Data
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1

Why is the following not giving me the correct count [ use sample_training ]

db.trips.find ( { “start station location” : { “$elemMatch” : { “coordinates.0”: { “$lt”: -74.00 }} } } ).count()
0

while this command gives correct count

db.trips.find ( { “start station location.coordinates.0” : { “$lt” : -74.00 } }).count()
1928

Thanks

2

The operator $elemMatch is for array. The field start station location is an object. However, start station location.coordinates is an array.

3

Thanks, Steve for pointing that out.

Just want to understand the use of $elemMatch.

I changed my query to

db.trips.find ( { “start station location.coordinates” : { “$elemMatch” : { “coordinates.0”: { “$lt”: -74.00 }} } } ).count()

It returns me 0.

I tried the above query by fully qualifying the element to start station location.coordinates.0 as follows:

db.trips.find ( { “start station location.coordinates” : { “$elemMatch” : { “start station location.coordinates.0”: { “$lt”: -74.00 }} } } ).count()

That does not work either.

Thanks for your attention.

4

I cannot explain better than the documentation.

See https://docs.mongodb.com/manual/reference/operator/query/elemMatch/ for description and examples.