MongoDB: Ordered matching of array entries - mongodb

Question

I have a document which looks like this:
"tokens":
[
{
"index": 1,
"word": "I",
"pos": "NNP",
},
{
"index": 2,
"word": "played",
"pos": "VBZ",
},
{
"index": 3,
"word": "football",
"pos": "IN",
}
]
And my query is:
db.test.find({
$and: [
{
'tokens.word': 'I'
},
{
tokens: {
$elemMatch: {
word: /f.*/,
pos: 'IN'
}
}
}
]
})
The output of my query is the document above. But the result should be no match in this case as I'm searching for
word: "I" followed by [word: /f.*/ and pos:'IN']
which doesn't match the tokens array in the document since the token I is followed by played and then football. However, in the query the order of the filters is different as the searching is started with
word: "I"
followed by
f.*
[football in this case].

Answer 1

The $and operator is a purely logical one where the order of the filter conditions does not play any role.
So the following queries are absolutely equivalent from a result set point of view:
$and: [
{ "a": 1 },
{ "b": 2 }
]
$and: [
{ "b": 2 },
{ "a": 1 }
]
The documentation states:
$and performs a logical AND operation on an array of two or more
expressions (e.g. , , etc.) and selects the
documents that satisfy all the expressions in the array. The $and
operator uses short-circuit evaluation. If the first expression (e.g.
) evaluates to false, MongoDB will not evaluate the
remaining expressions.
In your example, the "index1" entry matches the first filter "tokens.word": "I" and the "index3" document matches the second $elemMatch filter. So the document has to get returned.
UPDATE:
Here is an idea - more of a starting point really - for you to get closer to what you want:
db.collection.aggregate({
$addFields: { // create a temporary field
"differenceBetweenMatchPositions": { // called "differenceBetweenMatchPositions"
$subtract: [ // which holds the difference between
{ $indexOfArray: [ "$tokens.word", "I" ] }, // the index of first element matching first condition and
{ $min: { // the lowest index of
$filter: { // a filtered array
input: { $range: [ 0, { $size: "$tokens" } ] }, // that holds the indices [ 0, 1, 2, ..., n ] where n is the number of items in the "tokens" array - 1
as: "this", // which we want to access using "$$this"
cond: {
$let: {
vars: { "elem": { $arrayElemAt: [ "$tokens", "$$this" ] } }, // get the n-th element in our "tokens" array
in: {
$and: [
{ $eq: [ "$$elem.pos", "IN" ] }, // "pos" field must be "IN"
{ $eq: [ { $substrBytes: [ "$$elem.word", 0, 1 ] }, "f" ] } // 1st character of the "word" must be "f"
]
}
}
}
}
}
}]
}
}
}, {
$match: {
"differenceBetweenMatchPositions": { $lt: 0 } // we only want documents where the first condition is matched by an item in our array before the second one gets matched, too.
}
})