mongo aggregation - number of documents where field in one array is also in another one - mongodb

Question

I have a Movies collection
...
{
...
"cast":[ "First Actor", "Second Actor" ],
"directors":[ "First Director", "Second Director" ]
},
{
...
"cast": [ "Actor Director", "First Actor" ],
"directors": [ "Actor Director", "Firt Director" ]
}
...
Using aggregation framework I need to get number of documents where at least one value from directors array is also in a cast array. How could I achieve it?

Answer 1

You can use $setIntersection to find common entries in both arrays, then filter documents by $size of the result gt than 0 (means that at least one element is common to arrays), and finally use $count stage to count documents that match this condition.
-- EDIT : Add $addFields stage in case of no array present for cast or directors
In case of any document that doesn't contain cast or directors array, you will get an error for size waiting for an array and getting a null value.
In order to avoid this, you need to add an $addField stage to define empty array instead of null, for cast and directors.
Here's the query :
db.collection.aggregate([
{
$addFields: {
directors: {
$cond: {
if: {
$isArray: "$directors"
},
then: "$directors",
else: []
}
},
cast: {
$cond: {
if: {
$isArray: "$cast"
},
then: "$cast",
else: []
}
}
}
},
{
$match: {
$expr: {
$gt: [
{
$size: {
$setIntersection: [
"$cast",
"$directors"
]
}
},
0
]
}
}
},
{
$count: "have_common_value"
}
])
You can test it here