Find all embedded documents from manual reference in MongoDB - spring

Question

I use MongoDB and Spring Boot in a project. I used manual reference to point out a collection, My structure is as follows:
Reel collection
{
_id : "reel_id_1",
name: "reel 1",
category :[
{
_id : "category_id_1",
name: "category 1",
videos: ["video_id_1","video_id_2"]
}
]
}
Video collection
{
_id: "video_id_1", // first document
name: "mongo"
}
{
_id: "video_id_2", // seconddocument
name: "java"
}
Java classes are
#Document
#Data
public class Reel {
#Id
private ObjectId _id;
private String name;
List<Category> category;
}
#Data
public class Category {
#Id
private ObjectId _id=new ObjectId();
private String name;
Video videos;
}
#Document
#Data
public class Video {
#Id
private ObjectId _id = new ObjectId();
private String name;
}
I tried to join both document via mongoTemplate
public List<Reel> findById(ObjectId _id) {
LookupOperation lookupOperation = LookupOperation.newLookup()
.from("video")
.localField("category.videos")
.foreignField("_id")
.as("category.videos");
UnwindOperation unwindOperation = Aggregation.unwind("category");
Aggregation agg = newAggregation(unwindOperation,match(Criteria.where("_id").is(_id)),lookupOperation);
Aggregation aggregation = newAggregation(lookupOperation);
List<Reel> results = mongoTemplate.aggregate(aggregation, "reel", Reel.class).getMappedResults();
return results;
}
But it throws an error.
Failed to instantiate java.util.List using constructor NO_CONSTRUCTOR with arguments
But since I use "unwind", I created a new Entity UnwindReel and add Category category instead of List<Category> category. And used
List<UnwindReel> results = mongoTemplate.aggregate(aggregation, "reel", UnwindReel.class).getMappedResults();
It combines only first video (video_id_1) object. How can I get all objects inside videos array? Is there any method to fetch?

Answer 1

Your JSON stored in database has wrong structure. Your Reel class expects list of Category, but in database you have stored as nested object.
You need to add this stage just after $lookup
{
"$addFields": {
"category": {
"$map": {
"input": "$category.videos",
"in": {
"videos": "$$this"
}
}
}
}
}
Java code
public List<Reel> findById(String _id) {
Aggregation aggregation = Aggregation.newAggregation(
Aggregation.match(Criteria.where("_id").is(_id)),
Aggregation.lookup(mongoTemplate.getCollectionName(Video.class), "category.videos", "_id", "category.videos"),
new AggregationOperation() {
#Override
public Document toDocument(AggregationOperationContext context) {
return new Document("$addFields",
new Document("category", new Document("$map", new Document("input", "$category.videos")
.append("in", new Document("videos", "$$this")))));
}
})
.withOptions(AggregationOptions.builder().allowDiskUse(Boolean.TRUE).build());
LOG.debug(
aggregation.toString().replaceAll("__collection__", mongoTemplate.getCollectionName(Reel.class)));
return mongoTemplate.aggregate(aggregation, mongoTemplate.getCollectionName(Reel.class), Reel.class)
.getMappedResults();
}
Recomendations
Do not hard-code collection name, use better mongoTemplate.getCollectionName method
Always log aggregation pipeline before performing, it helps debugging.
If your collection will grow in the future, use {allowDiskUse: true} MongoDb aggregation option.