MongoDB query of array of documents returning all elements - json

Question

I have a collection called 'cupcakes' that was loaded from a GeoJSON file using mongoimport. The GeoJSON can be found here, and is a known valid JSON file. This part seems to have gone to plan, and doing:
use cupcakes
db.cupcakes.find({ })
This pulls back all the features as expected. However, I am trying to do a query that will return only the features that have gluten free set to "no".
I have tried the instructions as per the documentation around querying arrays of documents.
db.cupcakes.find({features: { $elemMatch: { "properties.gluten free":"no"}}}).pretty()
But as far as I can tell this appears to be returning all of the features as opposed to just those that are gluten free establishments.
Essentially I am wanting to pull back all the features where "properties.gluten free" of the features is set to "no" (or "yes" for that matter).
A sample output might be:
{
"type": "Feature",
"geometry": {
"type": "Point",
"coordinates": [
-122.70069837570189,
45.53570881624427
]
},
"properties": {
"name": "Le Cookie Monkey",
"address": "1902 NW 24th Avenue",
"website": "http://www.lecookiemonkey.com/",
"gluten free": "no",
"open1": "Tuesday - Friday, 9am - 3pm",
"open2": "Saturday, 9am - 2pm"
}
},
{
"type": "Feature",
"geometry": {
"type": "Point",
"coordinates": [
-122.6375323534012,
45.5219957171348
]
},
"properties": {
"name": "Crema Coffee + Bakery",
"address": "2728 SE Ankeny Street",
"website": "http://www.cremabakery.com/",
"gluten free": "no",
"open1": "Monday - Sunday, 7am - 6pm"
}
},
{
"type": "Feature",
"geometry": {
"type": "Point",
"coordinates": [
-122.60960519313811,
45.472775425296014
]
},
"properties": {
"name": "Mehri's Bakery & Deli",
"address": "6923 SE 52nd Avenue",
"website": "http://www.mehris.com/",
"gluten free": "no",
"open1": "Monday - Friday, 7am - 7pm",
"open2": "Saturday, 8am - 5pm",
"open3": "Sunday, 8am - 2pm"
}
},
... and so on until all the features where "gluten free" : "no" are returned.
What am I missing here? Thanks in advance.

Answer 1

You forgot to add the positional $ operator in your projection which limits the contents of the features array from the query results to contain only the first element matching the query document as follows:
db.cupcakes.find(
{ "features.properties.gluten free": "no" },
{ "features.$": 1, "_id": 0 }
);
Sample Output:
/* 0 */
{
"features" : [
{
"type" : "Feature",
"geometry" : {
"type" : "Point",
"coordinates" : [
-122.653357386589,
45.51208367658516
]
},
"properties" : {
"name" : "Hungry Heart Cupcakes",
"address" : "1212 SE Hawthorne Boulevard",
"website" : "http://www.hungryheartcupcakes.com",
"gluten free" : "no",
"open1" : "Monday - Sunday, 11am - 9pm"
}
}
]
}
To pull back all the features where gluten free is equal to no, the $redact pipeline operator in the aggregation framework should work for you in this case. This will recursively descend through the document structure and do some actions based on an evaluation of specified conditions at each level. The map() method will then produce the final array of objects as per expectation. The concept can be a bit tricky to grasp but the following example demonstrates this:
db.cupcakes.aggregate([
{
"$match": { "features.properties.gluten free" : "no" }
},
{
"$redact": {
"$cond": {
"if": {
"$eq": [ "$gluten free", "yes" ]
},
"then": "$$PRUNE",
"else": "$$DESCEND"
}
}
}
]).map(function(doc){ return doc.features; });