Unwind 3 arrays in MongoDB - arrays

Question

MongoDB collection data with multiple arrays:
{
"_id": ObjectId("61aa6bf1742b00f59b894eb7"),
"first": ["abc", "def", "ghi"],
"last": ["rst", "uvw", "xyz"],
"numb": ["12", "34", "56"]
}
Expected output where the data in the arrays should be in this format:
{
"first": "abc",
"last": "rst",
"numb": "12"
},
{
"first": "def",
"last": "uvw",
"numb": "34"
},
{
"first": "ghi",
"last": "xyz",
"numb": "56"
}

Answer 1

You can make use of $zip to "transpose" multiple arrays (as many as you'd like actually):
// {
// first: ["abc", "def", "ghi"],
// last: ["rst", "uvw", "xyz"],
// numb: ["12", "34", "56"]
// }
db.collection.aggregate([
{ $project: { x: { $zip: { inputs: ["$first", "$last", "$numb"] } } } },
// { x: [["abc", "rst", "12"], ["def", "uvw", "34"], ["ghi", "xyz", "56" ]] }
{ $unwind: "$x" },
// { x: [ "abc", "rst", "12" ] }
// { x: [ "def", "uvw", "34" ] }
// { x: [ "ghi", "xyz", "56" ] }
{ $replaceWith: {
$arrayToObject: { $zip: { inputs: [["first", "last", "numb"], "$x"] } }
}}
])
// { first: "abc", last: "rst", numb: "12" }
// { first: "def", last: "uvw", numb: "34" }
// { first: "ghi", last: "xyz", numb: "56" }
This:
zips the 3 arrays such that elements at the same index will get grouped into the same sub-array.
$unwinds (explodes/flattens) those sub-arrays.
transforms the resulting arrays into objects to fit your expected output format:
by $zipping (again!) the keys we want to associate with the array's values (the keys: ["first", "last", "numb"] and the values: "$x")
and $replaceWith the current document with the result of the $zip.
Note that prior to Mongo 4.2, you can use $replaceRoot instead of $replaceWith.

Answer 2

Query
map on indexes to combine the same index members to 1 document
keeps the _id also to know from which document those came from
and the index to sort after
for each index take the element from each array
unwind
sort by _id and index to get the results sorted like it was in the arrays
*indexes are computed using the biggest array, to be safe, in case you already know that all are the same size, you can replace the :
{"$max": [{"$size": "$first"}, {"$size": "$last"}, {"$size": "$numb"}]}
with the size of any array for example(we need the biggest to work):
{"$size": "$first"}
Test code here
aggregate(
[{"$project":
{"data":
{"$map":
{"input":
{"$range":
[0,
{"$max":
[{"$size": "$first"}, {"$size": "$last"}, {"$size": "$numb"}]}]},
"in":
{"_id": "$_id",
"index": "$$this",
"first": {"$arrayElemAt": ["$first", "$$this"]},
"last": {"$arrayElemAt": ["$last", "$$this"]},
"numb": {"$arrayElemAt": ["$numb", "$$this"]}}}}}},
{"$unwind": {"path": "$data"}},
{"$replaceRoot": {"newRoot": "$data"}},
{"$sort": {"_id": 1, "index": 1}},
{"$unset": ["index"]}])