PHP - MongoDB: Retrieve Fields from Sub Document with Field Names in Array - php

Question

Consider an Array
$lettersArray = [A,C,E,G]
and my MongoDB Collection has the following structure.
{
Collection : {
letters:{
A:{...},
B:{...},
...
Z:{...}
}
}
}
Consider that the Letter Sub document is a part of a larger collection. Hence I am using Aggregation.
Right Now I have tried to project -
['$Collection.letters' => ['$elemMatch' => ['$in' => $lettersArray]]
and also tried
['Letters' => ['$in' => [$lettersArray,'$Collection.letters']]
But it didn't work.
In the End, I want result like the following:
[
Collection => [
letters => [
A => [...],
C => [...],
E => [...],
G => [...]
]
]
]
Is there any way to do this?

Answer 1

In PHP you can use array_combine with array_fill to create the empty arrays.
$lettersArray = ['A','C','E','G'];
$letters = array_combine($lettersArray, array_fill(0,count($lettersArray), []));
Array_fill creates an array from index 0, to the count of items in $lettersArray with the contents []
Output:
array(4) {
["A"]=>
array(0) {
}
["C"]=>
array(0) {
}
["E"]=>
array(0) {
}
["G"]=>
array(0) {
}
}
https://3v4l.org/TeoFv

Answer 2

I think you are mistaken in the way you are trying to access the documents' information.
If you take a look at your MongoDB document, you will see that it is in fact not an array, so you should not use $elemMatch to project these fields, but simple projections. In your case, you should project in this way:
[
'$project' => [
'Collection.letters.A' => 1,
'Collection.letters.C' => 1,
'Collection.letters.E' => 1,
'Collection.letters.G' => 1
]
]
By the way, you don't need to use the aggregation framework to compute such a projection. You could just use find(), and use the projection in the options, which are the functions' second argument:
myCollection->find(query, [
'projection' => [
'Collection.letters.A' => 1,
'Collection.letters.C' => 1,
'Collection.letters.E' => 1,
'Collection.letters.G' => 1
]
]);
Cheers,
Charles