Cannot use Nested VariableOperators.mapItemsOf in Spring Data MongoDb - mongodb

Question

I'm forced to use the aggregation framework and the project operation of Spring Data MongoDb.
What I'd like to do is creating an array of object as a result of a project operation.
Considering this intermediate aggregation result:
{
"processes": [
{
"id": "101a",
"assignees": [
{
"id": "201a",
"username": "carl93"
},
{
"id": "202a",
"username": "susan"
}
]
},
{
"id": "101b",
"assignees": [
{
"id": "201a",
"username": "carl93"
},
{
"id": "202a",
"username": "susan"
}
]
}
]
}
I'm trying to get for each process, all the assignee usernames and ids. Hence, what I want to obtain is something like this:
[
{
"results": [
{
"id": "201a",
"value": "carl93",
"parentObjectId": "101a"
},
{
"id": "202a",
"value": "susan",
"parentObjectId": "101a"
},
{
"id": "201a",
"value": "carl93",
"parentObjectId": "101b"
},
{
"id": "202a",
"value": "susan",
"parentObjectId": "101b"
}
]
}
]
To reach this goal I'm using 2 nested VariableOperators.mapItemsOf obtaining:
org.springframework.data.mapping.MappingException: Cannot convert [Document{{id= 201a, value= carl93, parentObjectId= 101a}}, Document{{id= 202a, value = susan, parentObjectId= 101a}}]
of type class java.util.ArrayList into an instance of class java.lang.Object!
Implement a custom Converter<class java.util.ArrayList, class java.lang.Object> and register it with the CustomConversions.
Here's the code that I'm currently using:
new ProjectionOperation().and(
VariableOperators.mapItemsOf("processes")
.as("pr")
.andApply(
VariableOperators.mapItemsOf("$pr.ownership.assignees")
.as("ass")
.andApply(aggregationOperationContext -> {
Document document = new Document();
document.append("id", "$$ass.id");
document.append("value", "$$ass.username");
document.append("parentObjectId", "$$pr.id");
return document;
})
)
).as("results");
The code produces this:
[
[
{
"id": "201a",
"value": "carl93",
"parentObjectId": "101a"
},
{
"id": "202a",
"value": "susan",
"parentObjectId": "101a"
}
],
[
{
"id": "201a",
"value": "carl93",
"parentObjectId": "101b"
},
{
"id": "202a",
"value": "susan",
"parentObjectId": "101b"
}
]
]
As you can see there are 2 nested arrays, [[],[]]. This is the reason why the exception is thrown.
Nevertheless what I want to obtain is just one array, adding all the objects in it (possibly without duplicates or null values). I've tried the addToSet operator and other aggregtion operators, without any success.

Answer 1

Use $reduce with $concatArrays to join the arrays.
new ProjectionOperation().and(
ArrayOperators.arrayOf("processes")
.reduce(ArrayOperators.ConcatArrays.arrayOf("$$value").concat(
VariableOperators.mapItemsOf("$$this.ownership.assignees")
.as("ass")
.andApply(aggregationOperationContext -> {
Document document = new Document();
document.append("id", "$$ass.id");
document.append("value", "$$ass.username");
document.append("parentObjectId", "$$this.id");
return document;
})
)).startingWith(Arrays.asList())
).as("results");