How to add a sub-document field with a condition in MongoDB? - mongodb

Question

I am trying to add a new subField with a condition.
In the case the field already exists, I don't overwrite it.
In the case the condition is not fulfilled, I don't want to add the parent object.
Here is my collection :
{type: "A", object: {a: "", b: "foo"}},
{type: "A", object: {a: ""}},
{type: "A"},
{type: "B"}
Here is my aggregate :
{
$addFields: {
"object.b": {
$cond: {
if: {$eq: ["$type","A"]},
then: {$ifNull: ["$object.b", "bar"]},
else: "$DROP"
}
}
}
}
$DROP is not an aggregate command, but in the else case I don't want to add the new field.
It will not create the b field, but the parent object remains.
Here is my current result :
{type: "A", "object": {a: "", b: "foo"}},
{type: "A", "object": {a: "", b: "bar"}},
{type: "A", "object": {b: "bar"}},
{type: "B", "object": {}},
Here is what I want :
{type: "A", object: {a: "", b: "foo"}},
{type: "A", object: {a: "", b: "bar"}},
{type: "A", object: {b: "bar"}},
{type: "B"}
Your help will be highly appreciated.

Answer 1

This aggregate query will give you the desired result:
db.collection.aggregate([
{
$addFields: {
object: {
$cond: {
if: { $eq: [ "$type", "A" ] },
then: {
$mergeObjects: [
"$object",
{ b: { $ifNull: [ "$object.b", "bar" ] } }
]
},
else: "$$REMOVE"
}
}
}
}
])
Note the $$REMOVE is a aggregation system variable.

Answer 2

When a $set adds a path, all path is added, even if you end up to $$REMOVE this will affect only the last of the path, the rest would be already added see example
Query
$set with switch case starting from object
if object doesn't exist
if type A add {object :{"b" : "bar"}}
else $$REMOVE
$type = "A" AND (not-value "$object.b")
add {"b" : "bar"} (this case covers also the case object:null)
else
keep old value (another type, or b had value)
*Maybe it could be smaller but we check many things(see the example for all cases of data)
object exists/null/not exists
type A/not
b exists/null/value
Test code here
db.collection.aggregate([
{
"$set": {
"object": {
"$switch": {
"branches": [
{
"case": {
"$eq": [
{
"$type": "$object"
},
"missing"
]
},
"then": {
"$cond": [
{
"$eq": [
"$type",
"A"
]
},
{
"b": "bar"
},
"$$REMOVE"
]
}
},
{
"case": {
"$and": [
{
"$eq": [
"$type",
"A"
]
},
{
"$or": [
{
"$eq": [
"$object.b",
null
]
},
{
"$eq": [
{
"$type": "$object.b"
},
"missing"
]
}
]
}
]
},
"then": {
"$mergeObjects": [
"$object",
{
"b": "bar"
}
]
}
}
],
"default": "$object"
}
}
}
}
])