How to use '$let' in MongoDB Aggregation Query in Scala? - mongodb

Question

I am trying to write a mongoDB aggregation query in Scala.
How do I write Scala code to use "$let" in '$project' stage?
I am wondering if Variable should be used. Not sure how?
'$project': {
'myprojitem' :{
'$let': {
'vars' : { 'myVariable1': { '$or': [...] } }
'in' : {
'$cond': [
'$$myVariable1',
{ ... },
{ ... },
]
}
}

Answer 1

I figured out the answer. Hopefully it helps someone.
val doc : Document = Document("{
'$let': {
'vars' : { 'myVariable1': { '$or': [...] } },
'in' : { '$cond': ['$$myVariable1',{ ... },{ ... } ]
}
}")
var pipeline = mutable.Buffer[Bson]()
pipeline += Aggregates.project(Projections.fields(
Projections.computed("myprojitem",doc)
))
Basically, every { name : expression } can be written as :
Document("name" -> expression)
Or
Document( "{name : expression}")

Answer 2

$let is used to bind variables together to a results obj. The syntax follows the rule:
{
$let:
{
vars: { <var1>: <expression>},
in: <expression>
}
}
for mere details you should take a look at $let (aggregation) definition from mongodb manual
Here is a text book example just to make more sense:
Consider the following data:
{ _id: 1, price: 10, tax: 0.50, applyDiscount: true }
{ _id: 2, price: 10, tax: 0.25, applyDiscount: false }
And imagine that we want to generate a result for the finalTotal in a way that:
Where Disc = 10% if applyDiscount: true and 0 otherwise.
So we need now to create the aggregation on the data to construct this equation. So we can get a results like:
{ _id: 1, finalTotal: 9.45 }
{ _id: 2, finalTotal: 10.25 }
We can do this by doing:
$project: {
finalTotal: {
$let: {
vars: {
total: { $add: [ '$price', '$tax' ] },
discounted: { $cond: { if: '$applyDiscount', then: (0.9, else: 1 } }
},
in: { $multiply: [ "$$total", "$$discounted" ] }
}
}
}
We can break this down:
Step 1. adding price to tax together to a variable called total
total: { $add: [ '$price', '$tax' ] },
Step 2. transforming the condition in numbers (variable discounted)
discounted: { $cond: { if: '$applyDiscount', then: 0.9, else: 1 } }
Step 3. performing the operation $multiply operation between the constructed $$total and $$discounted
in: { $multiply: [ "$$total", "$$discounted" ] }