MongoDB query $lookup with prefix - mongodb

Question

I have two collection rounds and summaries
A record in rounds looks like
{
"_id": "2018-04",
"name": "Round 2018-04"
}
A record in summaries look like
{
"phase": "round:2018-04",
"userId": NumberLong(66325),
}
I want to query summaries and lookup into rounds joining based on phase of summaries into _id of rounds
PROBLEM: It will be pretty simple if there was no prefix of round: in phase.
Is there anyway to do this?
This is my code so far.
$cursor = $this->mongo->selectCollection('summaries')->aggregate([
array('$match' => []),
array(
'$lookup' => [
'from' => 'rounds',
'localField' => 'phase',
'foreignField' => '_id',
'as' => 'roundDetail'
]
),
array(
'$unwind' => '$roundDetail',
),
array(
'$project' => [
'userId' => 1,
'roundDetail.name' => 1
]
)
]);
MongoDB version 3.4.16

Answer 1

You can use substrBytes to remove characters from the string.
$cursor = $this->mongo->selectCollection('summaries')->aggregate([
array('$match' => []),
array('$addFields' => [ 'phase' => [ '$substrBytes' => [ '$phase', 6, 7 ] ] ] ),
array(
'$lookup' => [
'from' => 'rounds',
'localField' => 'phase',
'foreignField' => '_id',
'as' => 'roundDetail'
]
),
array(
'$unwind' => '$roundDetail',
),
array(
'$project' => [
'userId' => 1,
'roundDetail.name' => 1
]
)
])