How to address arrays with mongodb Set Operators - mongodb

Question

Using the example zipcodes collection, I have a query like this:
db.zipcodes.aggregate([
{ "$match": {"state": {"$in": ["PA","NY"]}}},
{ "$group": { "_id": { "city": "$city" }, "ZipsPerCity": {"$addToSet": "$_id"}}},
{ "$match": { "ZipsPerCity" : { "$size": 2 }}},
]).pretty()
This is just an example that looks for cities (in the state of NY and PA) that have 2 zipcodes:
{
"_id" : {
"city" : "BETHLEHEM"
},
"ZipsPerCity" : [
"18018",
"18015"
]
}
{
"_id" : {
"city" : "BEAVER SPRINGS"
},
"ZipsPerCity" : [
"17843",
"17812"
]
}
Now suppose that I want to compare "BEAVER SPRINGS" zip codes to "BETHLEHEM" zip codes, using the "$setDifference" set operator? I tried using the "$setDifference" operator in a $project operator, like this:
db.zipcodes.aggregate([
{ "$match": { "state": {"$in": ["PA","NY"]}}},
{ "$group": { "_id: {city : "$city"},"ZipsPerCity": {$addToSet: "$_id"}}},
{ "$match": { "ZipsPerCity" : { $size: 2 }}},
{ "$project": {
"int": { "$setDifference":[
"$_id.city.BETHLEHEM.ZipsPerCity",
"$_id.city.BEAVER SPRINGS.ZipsPerCity"
]}
}}
]).pretty()
That doesn't even look right, let alone produce results. No errors though.
How would you refer to a couple of arrays built using $addToSet like this, using $setDifference (or any of the set operators)?

Answer 1

The first thing about what you are trying to do here is that the arrays you want to compare are actually in two different documents. All of the aggregation framework operators in fact work on only one document at a time, with the exception of $group which is meant to "aggregate" documents and possibly $unwind which essentially turns one document into many.
In order to compare you would need the data to occur in one document, or at least be "paired" in some way. So there is a technique to do that:
db.zipcodes.aggregate([
{ "$match": {"state": { "$in": [ "PA","NY" ] } }},
{ "$group": {
"_id": "$city",
"ZipsPerCity": { "$addToSet": "$_id"}
}},
{ "$match": { "ZipsPerCity" : { "$size": 2 } }},
{ "$group": {
"_id": null,
"A": { "$min": {
"$cond": [
{ "$eq": [ "$_id", "BETHLEHEM" ] },
{ "city": "$_id", "ZipsPerCity": "$ZipsPerCity" },
false
]
}},
"B": { "$min": {
"$cond": [
{ "$eq": [ "$_id", "BEAVER SPRINGS" ] },
{ "city": "$_id", "ZipsPerCity": "$ZipsPerCity" },
false
]
}}
}},
{ "$project": {
"A": 1,
"B": 1,
"C": { "$setDifference": [ "$A.ZipsPerCity", "$B.ZipsPerCity" ] }
}}
])
That is a little contrived and I am well aware that the actual result set has more than two cities, but the point it to illustrate that the arrays/sets sent to the "set operators" such as $setDifference need to be in the same document.
The result here compares the "left" array with the "right" array, returning the members from the "left" that are different to the "right". Both sets are unique here with no overlap so the results should be expected:
{
"_id" : null,
"A" : {
"city" : "BETHLEHEM",
"ZipsPerCity" : [
"18018",
"18015"
]
},
"B" : {
"city" : "BEAVER SPRINGS",
"ZipsPerCity" : [
"17843",
"17812"
]
},
"C" : [
"18018",
"18015"
]
}
This is really better illustrated with actual "sets" with common members. So this document:
{ "A" : [ "A", "A", "B", "C", "D" ], "B" : [ "B", "C" ] }
Responds to $setDifference:
{ "C" : [ "A", "D" ] }
And $setEquals:
{ "C" : false }
$setIntersection:
{ "C" : [ "B", "C" ] }
$setUnion:
{ "C" : [ "B", "D", "C", "A" ] }
$setIsSubSet reversing the order to $B, $A:
{ "C" : true }
The other set operators $anyElementTrue and $allElementsTrue are likely most useful when used along with the $map operator which can re-shape arrays and evaluate conditions against each element.
A very good usage of $map is alongside $setDifference, where you can "filter" array contents without using $unwind:
db.arrays.aggregate([
{ "$project": {
"A": {
"$setDifference": [
{
"$map": {
"input": "$A",
"as": "el",
"in": {
"$cond": [
{ "$eq": [ "$$el", "A" ] },
"$$el",
false
]
}
}
},
[false]
]
}
}}
])
That can be very handy when you have a lot of results in the pipeline and you do not want to "expand" out all of those results by "unwinding" the array. But note that this is a "set" and as such only one element matching "A" is returned:
{ "A" : ["A"] }
So the things to keep in mind here are that you:
Operate only within the "same" document at a time
The results are generally "sets" and that means they are both "unique" and "un-ordered" as a result.
Overall that should be a decent run-down on what the set operators are and how you use them.