MongoDB Realm - Filter Queries - mongodb

Question

In the scheme section where I connect the realm to the MongoDB collections there is a tab to add a filter query to filter the results being synced. I added a filter but every time I query on the app to load the data I get all the documents instead of filtered documents.
How can I filter the data the app receives?
Collection Schema:
{
"title": "Group",
"required": [
"_id",
"cDate",
"name",
"info",
"isPublic",
"joinWithRequest",
"partition"
],
"properties": {
"_id": {
"bsonType": "objectId"
},
"admins": {
"bsonType": "array",
"items": {
"bsonType": "objectId"
}
},
"members": {
"bsonType": "array",
"items": {
"bsonType": "objectId"
}
},
"photoItems": {
"bsonType": "array",
"items": {
"bsonType": "object",
"title": "Item",
"properties": {
"id": {
"type": "string"
},
"cDate": {
"bsonType": "date"
}
},
"required": [
"id",
"cDate"
]
}
},
"videoItems": {
"bsonType": "array",
"items": {
"bsonType": "object",
"title": "Item",
"properties": {
"id": {
"type": "string"
},
"cDate": {
"bsonType": "date"
}
},
"required": [
"id",
"cDate"
]
}
},
"cDate": {
"bsonType": "date"
},
"partition": {
"bsonType": "string"
},
"name": {
"bsonType": "string"
},
"info": {
"bsonType": "string"
},
"icon": {
"bsonType": "string"
},
"isPublic": {
"bsonType": "bool"
},
"joinWithRequest": {
"bsonType": "bool"
}
}
}
Filter:
Client Query:
let groups = realm.objects(Group.self).sorted(byKeyPath: "cDate")

Answer 1

What you're asking about is (was) called a query based sync. While it limited the amount of data sync'd it also was limited by the number of users, system resources etc.
At this time query based sync is no longer supported in MongoDB Realm
However, you can get much of the same functionality by leveraging partitions - which are required for sync'ing anyway. Let me give a high level example.
Suppose you have a 'posts' app with users and then posts done by users of specific groups. For this example, this user belongs to Group_0 and Group_1. Your user object may look like this
class UserClass: Object {
#objc dynamic var _id: //a users uid
#objc dynamic var user_name = ""
let groupPartitionList = List<GroupClass>() //will populate with Group_0 and Group_1
}
and a post may look like this
class PostClass: Object {
#objc dynamic var _id: ObjectId = ObjectId.generate()
#objc dynamic var _partitionKey: GroupPartition = ""
#objc dynamic var title = ""
#obcj dynamic var post = ""
override static func primaryKey() -> String? {
return "_id"
}
}
Suppose there are 50 different groups.
Upon the user authenticating and the user object populating, you would then know the user belongs to Group_0 and Group_1 which could be displayed in a popup menu - defaulting to the first group, Group_0.
Once the user logs in, get just the Group_0 posts
let whichPartition = myUserObject.groupPartitionList[0] //object at index 0 = Group_0
let config = user.configuration(partitionValue: whichPartition)
Realm.asyncOpen(configuration: config) { result in
...
let group0Posts = realm.objects(PostClass.self) //loads just the posts Group_0
}
As you can see, there is no query: the data is limited by the partition key e.g this will not read posts from any other group - only posts with a partition key of Group_0
Likewise, if the user changes the popup to Group_1, only posts with partition key of Group_1 will sync and no other. So even though there are 50 groups and thousands of posts, you will only download and sync Group_0's and Group_1's via the partition value.
You can augment this further by using a server filter (as mentioned in the comments to the question) to withhold certain data so the amount of data being queried at the server level is smaller.
For example, you could add a filter to ignore any posts that are older than 5 years. That will significantly speed up queries by reducing the amount of data being queried (which effectively could reduce the number of results because those are ignored)
There's some excellent reading in the MongoDB Realm Docs Partition Atlas Data Into Realms.