Query for documents where match contiguous array elements - mongodb

Question

I have a MongoDB collection with documents in the following format:
{ "_id" : 1, "tokens": [ "I", "have", "a", "dream" ] },
{ "_id" : 2, "tokens": [ "dream", "a", "little", "dream" ] },
{ "_id" : 3, "tokens": [ "dream", "a", "dream" ] },
{ "_id" : 4, "tokens": [ "a" , "little", "dream" ] },
...
I need to get all doucuments which "tokens" include contiguous array elements: "a", "dream".
So, the following are matched doucuments:
{ "_id" : 1, "tokens": [ "I", "have", "a", "dream" ] },
{ "_id" : 3, "tokens": [ "dream", "a", "dream" ] },
Is there a way to get the right results?

Answer 1

A trick that is to have a regexp.
$match to get the all documents which has $all array input
$addFields to have a duplicate the tokens and input array
$reduce helps to concat all string joining -
$regexMatch to match both strings
$match to eliminate unwanted data
$project to get necessary fields only
The code is
[{
$match: {
tokens: { $all: ["a", "dream"] }
}
}, {
$addFields: {
duplicate: "$tokens",
inputData: ["a", "dream"]
}
}, {
$addFields: {
duplicate: {
$reduce: {
input: "$duplicate",
initialValue: "",
in: { $concat: ["$$value", "-", "$$this"] }
}
},
inputData: {
$reduce: {
input: "$inputData",
initialValue: "",
in: { $concat: ["$$value", "-", "$$this"] }
}
}
}
}, {
$addFields: {
match: {
$regexMatch: { input: "$duplicate", regex: '$inputData' }
}
}
}, {
$match: {
match: true
}
}, {
$project: { _id: 1, tokens: 1 }
}]
Working Mongo playground
Note: Do check multiple scenarios although its working for this scenario