MongoTemplate aggregate projection, getting an exception when map array properties - mongodb

Question

I have two objects and two projection objects like below,
Main Objects
public class MainItem {
private String name;
private List<subItem> subItems;
}
public class subItem {
private String id;
private String groupId;
private String displayName;
private Status status;
}
Projection objects
public class MainItemLight {
private String name;
private List<subItemLight> subItemList;
}
public class subItemLight {
private String id;
private String name;
}
I'm trying to map Main objects to projection objects and return a list of MainItemLight objects. Below is my code,
mongoTemplate.aggregate(
newAggregation(project("name")
.and("subItems").as("subItemList")
.and("subItems.displayName").as("subItemList.name")
),
"MyCollection", MainItemLight.class).getMappedResults();
When I'm trying to map subItems.displayName to subItemList.name I get bellow exception,
Command failed with error 40176 (Location40176): 'Invalid $project
:: caused by :: specification contains two conflicting paths.
Cannot specify both 'subItemList.name' and 'subItemList'
Any idea how to fix this?

Answer 1

You need to do it like this:
db.collection.aggregate([
{
$project: {
name: 1,
subItemList: {
$map: {
input: "$subItems",
as: "item",
in: {
id: "$$item.id",
name: "$$item.displayName"
}
}
}
}
}
])
MongoPlayground
MongoTemplate
Aggregation agg = newAggregation(project("name").and(
VariableOperators.Map.itemsOf("subItems").as("item").andApply(
doc -> new Document()
.append("id", "$$item.id")
.append("name", "$$this.displayName")
)
).as("subItemList"));
Note: The $map implementation is not friendly in Spring Mongo, so wee need to implement it manually.
Source: SpringData mongoDB API for Aggregation $map