MongoDB - Query on the last element of an array?

Question

I know that MongoDB supports the syntax find{array.0.field:"value"}, but I specifically want to do this for the last element in the array, which means I don't know the index. Is there some kind of operator for this, or am I out of luck?

EDIT: To clarify, I want find() to only return documents where a field in the last element of an array matches a specific value.

Solution 1

I posted on the official Mongo Google group here, and got an answer from their staff. It appears that what I'm looking for isn't possible. I'm going to just use a different schema approach.

Solution 2

In 3.2 this is possible. First project so that myField contains only the last element, and then match on myField.

db.collection.aggregate([
   { $project: { id: 1, myField: { $slice: [ "$myField", -1 ] } } },
   { $match: { myField: "myValue" } }
]);

Solution 3

You can use $expr ( 3.6 mongo version operator ) to use aggregation functions in regular query.

Compare query operators vs aggregation comparison operators.

For scalar arrays

db.col.find({$expr: {$gt: [{$arrayElemAt: ["$array", -1]}, value]}})

For embedded arrays - Use $arrayElemAt expression with dot notation to project last element.

db.col.find({$expr: {$gt: [{"$arrayElemAt": ["$array.field", -1]}, value]}})

Spring @Query code

@Query("{$expr:{$gt:[{$arrayElemAt:[\"$array\", -1]}, ?0]}}")
ReturnType MethodName(ArgType arg);

Solution 4

use $slice.

db.collection.find( {}, { array_field: { $slice: -1 } } )

Editing: You can make use of { <field>: { $elemMatch: { <query1>, <query2>, ... } } } to find a match.

But it won't give exactly what you are looking for. I don't think that is possible in mongoDB yet.

Solution 5

Starting Mongo 4.4, the aggregation operator $last can be used to access the last element of an array:

---

For instance, within a find query:

// { "myArray": ["A", "B", "C"] }
// { "myArray": ["D"] }
db.collection.find({ $expr: { $eq: [{ $last: "$myArray" }, "C"] } })
// { "myArray": ["A", "B", "C"] }

---

Or within an aggregation query:

db.collection.aggregate([
  { $addFields: { last: { $last: "$myArray" } } },
  { $match: { last: "C" } }
])

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