Best way of doing lists that can be indexed efficiently in Mongo - mongodb

Question

I have a collection with documents that can either look like this (option A):
{
"my_list": [
{ "id": "A", "other_data": 123 },
{ "id": "B", "other_data": 456 },
{ "id": "C", "other_data": 789 },
]
}
or like this (option B):
{
"my_list": {
"A": 123,
"B": 456,
"C": 789,
}
}
Question is: which one is more efficient for doing queries such as: fetch me all documents that have id 'B' in 'my_list'?
Also, for the better option, how do you tell Mongo to create the relevant index?

Answer 1

Definitely the first one.
{
"my_list": [
{ "id": "A", "other_data": 123 },
{ "id": "B", "other_data": 456 },
{ "id": "C", "other_data": 789 },
]
}
MongoDB uses multikey indexes to index the content stored in arrays. If you index a field that holds an array value, MongoDB creates separate index entries for every element of the array. These multikey indexes allow queries to select documents that contain arrays by matching on element or elements of the arrays. MongoDB automatically determines whether to create a multikey index if the indexed field contains an array value; you do not need to explicitly specify the multikey type.
https://docs.mongodb.com/manual/indexes/#multikey-index
The second option it's object type. You need to create Single Field or Compound Index to use indexes.
Transform arrays into key:value store
MongoDB allows you to transform Multikey Index array as key:value store for during aggregation i.e.:
db.collection.aggregate([
{
$match: { "my_list.id" : "A" }
},
{
$project: {
my_list: {
$arrayToObject: {
$map: {
input: "$my_list",
in: {
k: "$$this.id",
v: "$$this.other_data"
}
}
}
}
}
}
])
If we take a look explain command, MongoDB uses indexes for efficient execution of queries.
{
"stages" : [
{
"$cursor" : {
"query" : {
"my_list.id" : "A"
},
"fields" : {
"my_list" : 1,
"_id" : 1
},
"queryPlanner" : {
"plannerVersion" : 1,
"namespace" : "test.collection",
"indexFilterSet" : false,
"parsedQuery" : {
"my_list.id" : {
"$eq" : "A"
}
},
"queryHash" : "599B2BF4",
"planCacheKey" : "48B2FCB0",
"winningPlan" : {
"stage" : "FETCH",
"inputStage" : {
"stage" : "IXSCAN",
"keyPattern" : {
"my_list.id" : 1.0
},
"indexName" : "my_list.id_1",
"isMultiKey" : true,
"multiKeyPaths" : {
"my_list.id" : [
"my_list"
]
},
"isUnique" : false,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 2,
"direction" : "forward",
"indexBounds" : {
"my_list.id" : [
"[\"A\", \"A\"]"
]
}
}
},
"rejectedPlans" : []
}
}
},
{
"$project" : {
"_id" : true,
"my_list" : {
"$arrayToObject" : [
{
"$map" : {
"input" : "$my_list",
"as" : "this",
"in" : {
"k" : "$$this.id",
"v" : "$$this.other_data"
}
}
}
]
}
}
}
],
"ok" : 1.0
}