Making a query on a mongo DB with ParseSwift - swift

Question

I have two collections in a Mongo DB.
Here is how a document looks in the first collection (MainCollection):
_id
:"mzWqPEDYRU"
TITLE
:"ZAZ: I want."
ownerID
:"lGutCBY52g"
accessKey
:"0kAd4TOmoK0"
_created_at
:2020-03-13T11:42:11.169+00:00
_updated_at
:2020-03-13T17:08:15.090+00:00
downloadCount
:2
And here is how it looks in the second collection (SecondCollection):
_id
:"07BOGA8bHG"
_p_unit
:"MainCollection$mzWqPEDYRU"
SENTENCE
:"I love nature peace and freedom."
Order
:5
ownerID
:"lGutCBY52g"
AUDIO
:"07067b5589d1edd1d907e96c1daf6da1_VOICE.bin"
_created_at
:2020-03-13T11:42:17.483+00:00
_updated_at
:2020-03-13T11:42:19.336+00:00
There is a parent children relationship between the first and the second collection. In the last document we can see the _p_unit field where the "mzWqPEDYRU" part points to the id of the parent in the first collection.
Though I finally get what I want, getting elements of the second collection with a given parent, it is not done how it should.
I have one problem making a selective query on SecondCollection. Here is the currently working code:
func theFunction(element: MainCollection) {
do {SecondCollection.query().find() {
result in
switch result {
case .success(let items):
print("items.count = \(items.count)")
var finalCount = 0
for item in items {
// Ignore useless elements:
if item.unit?.objectId != element.objectId! {continue}
finalCount += 1
/// .... Work with selected element.
}
print("finalCount = \(finalCount)")
case .failure(let error):
print("Error in \(#function): \(error)")
}
}
}
}
The way the above code is written works in the sense that I get the elements in SecondCollection I am interested in. But this trick inside the for loop to eliminate the non-needed element is not the way to go.
if item.unit?.objectId != element.objectId! {continue}
The filtering should happen in the query, with the line:
SecondCollection.query().find()
The problem is that everything I have tried failed. I did things like:
SecondCollection.query("unit" == element.objectId!).find()
with a zillion variations, but all with no luck.
Does anybody know the proper syntax?
In case this may be useful, here is how SecondCollection is declared:
struct SecondCollection: ParseObject,Identifiable,Equatable,Hashable {
// These fields are required for any Object.
var objectId: String?
var createdAt: Date?
var updatedAt: Date?
var ACL: ParseACL?
// Local properties.
var id:UUID {return UUID()}
var SENTENCE: String?,
Order: Int?,
ownerID: String?,
AUDIO: ParseFile?,
unit: Pointer<MainCollection>?
.......
}

Answer 1

First, I do not think that bringing all elements and then "eliminating" the ones you don't need is a good approach. That way you are retrieving a lot more data than needed and it is very inneficient.
I tried out your code but you did not make clear (at least for me) if you are using Pointers or Relations between those classes. The approach would be different for each one.
Which one are you using?
UPDATE: Hey there! I think I could make it work kinda the way you need it.
I created two classes ParentClass (name: string, age: number, children: relation to ChildClass):
struct ParentClass: ParseObject {
//: These are required for any Object.
var objectId: String?
var createdAt: Date?
var updatedAt: Date?
var ACL: ParseACL?
//: Your own properties.
var name: String?
var age: Int = 0
var children: ParseRelation<Self> {
ParseRelation(parent: self, key: "children", className: "ChildClass")
}
}
And ChildClass (name: string, age:number):
struct ChildClass: ParseObject {
//: These are required for any Object.
var objectId: String?
var createdAt: Date?
var updatedAt: Date?
var ACL: ParseACL?
//: Your own properties.
var name: String?
var age: Int = 0
}
Then I did a simple query to find the first Parent. You could use the find() method and bring all the ones that you need, but I did it this way to keep it simpler to explain:
let parentQuery = ParentClass.query()
parentQuery.first { result in
switch result {
case .success(let found):
print ("FOUND A PARENT!")
print(found.name!)
print(found.age)
print(found.children)
case .failure(let error):
print(error)
}
}
Now that I got the Parent (that has two children in my case), I used the query() method to generate the query on the ChildClass containing all the ChildClass' objects with the found parent:
do {
let childrenObject = ChildClass()
try found.children.query(childrenObject).find()
{ result in
switch result {
case .success(let allChildrenFromParent):
print("The following Children are part of the \(found.name!):")
for child in allChildrenFromParent {
print("\(child.name!) is a child of \(found.name!)")
}
case .failure(let error):
print("Error finding Children: \(error)")
}
}
} catch {
print("Error: \(error)")
}
And the whole code ended up like this:
let parentQuery = ParentClass.query()
parentQuery.first { result in
switch result {
case .success(let found):
print ("FOUND A PARENT!")
print(found.name!)
print(found.age)
print(found.children)
do {
let childrenObject = ChildClass()
try found.children.query(childrenObject).find()
{ result in
switch result {
case .success(let allChildrenFromParent):
print("The following Children are part of the \(found.name!):")
for child in allChildrenFromParent {
print("\(child.name!) is a child of \(found.name!)")
}
case .failure(let error):
print("Error finding Children: \(error)")
}
}
} catch {
print("Error: \(error)")
}
case .failure(let error):
print(error)
}
}