Calculate weight and skip a specific element Mongodb - Aggregation Framework - arrays

Question

I have a collection with array elements containing A,B or C values. I have to calculate a weight of each element value.
The logic of this weight is sample :
We give 1.0 as weight of the last element (dfferente to C), and others 0
If the last element of the array is C, we give it to the previous last element (different to C).
if there is 1 element in the array (A,B or C) we give it 1 as a weight.
This is how my collection looking like :
{
"_id" : ObjectId("5a535c48a4d86ed94a7e8618"),
"myArray" : [
{
"value" : "C"
},
{
"value" : "A
},
{
"value" : "C"
},
{
"value" : "B"
},
{
"value" : "A"
},
{
"value" : "A"
}
]
}
{
"_id" : ObjectId("5a535c48a4d86ed94a7e8619"),
"myArray" : [
{
"value" : "A"
},
{
"value" : "C"
},
{
"value" : "B"
},
{
"value" : "C"
},
{
"value" : "C"
}
]
}
I did some aggregation to make it happen but I can here just to affect 1 to the last (different to C)
and (if the last is C I give 1 to the last one -1) so I have to fixe the case of having C in the last and last-1 and last -2 .. last-n
so I have to fix it to affect 1 to the last one different to C
db.col.aggregate([{'$addFields':{
'myArray_temp':{
'$switch':{
'branches':[{
'case':{'$and':[{'$gt':[{'$size':'$myArray'},1]},{'$eq':[{'$arrayElemAt':['$myArray.value',-1]},'C']}]},
'then':{'$concatArrays':[
{'$map':{
'input':{'$slice':['$myArray',{'$subtract':[{'$size':'$myArray'},2]}]},
'as':'val',
'in':{'value':'$$val.value','weight':0 }
}},
[{'value':{'$arrayElemAt':['$myArray.value',-2]},'weight':1}],
[{'value':{'$arrayElemAt':['$myArray.value',-1]},'weight':0}]
]}
},
{
'case':{'$eq':[{'$size':'$myArray'},1]},
'then':{'$concatArrays':[
[{'value':{'$arrayElemAt':['$myArray.value',0]},'weight':1}]
]}
},
{
'case':{'$and':[{'$gt':[{'$size':'$myArray'},1]},{'$ne':[{'$arrayElemAt':['$myArray.value',-1]},'C']}]},
'then':{'$concatArrays':[
{'$map':{
'input':{'$slice':['$myArray',{'$subtract':[{'$size':'$myArray'},1]}]},
'as':'val',
'in':{'value':'$$val.value','weight':0 }
}},
[{'value':{'$arrayElemAt':['$myArray.value',-1]},'weight':1}]
]}
}
],
'default':{'$concatArrays':[
[{'value':{'$arrayElemAt':['$myArray.value',0]},'weight':1}]
]}
}
}
}}])
The results should be :
{
"_id" : ObjectId("5a535c48a4d86ed94a7e8618"),
"myArray" : [
{
"value" : "C",
"weight": 0
},
{
"value" : "A" ,
"weight": 0
},
{
"value" : "C",
"weight": 0
},
{
"value" : "B" ,
"weight": 0
},
{
"value" : "A",
"weight": 0
},
{
"value" : "A",
"weight": 1
}
]
}
{
"_id" : ObjectId("5a535c48a4d86ed94a7e8619"),
"total" : 4.5,
"myArray" : [
{
"value" : "A",
"weight": 0
},
{
"value" : "C",
"weight": 0
},
{
"value" : "B" ,
"weight": 1 // here we give 1 to the last differente to C
},
{
"value" : "C" ,
"weight": 0 // my code affect 1 here cause it find C in the last and affect to the last-1 element.
},
{
"value" : "C" ,
"weight": 0
}
]
}
We have to skip all the last (C) elements and give the 1 weight to the last not-C element.
Thank you in advance!

Answer 1

Longest aggregation in my career but seems to be working:
db.collection.aggregate([
{
$addFields : {
size: { $size: "$myArray" },
reversed: {
$reverseArray: "$myArray"
}
}
},
{
$addFields: {
otherThanC: {
$filter: {
input: "$reversed",
as: "item",
cond: { $ne: [ "$$item.value", "C" ] }
}
}
}
},
{
$addFields: {
firstOtherThanCIndex : {
$indexOfArray: [ "$reversed", { $arrayElemAt: [ "$otherThanC", 0 ] } ]
}
}
},
{
$unwind: {
path: "$reversed",
includeArrayIndex: "arrayIndex"
}
},
{
$addFields: {
weight: {
$switch: {
branches: [
{ case: { $eq: [ "$size", 1 ] }, then: 1 },
{ case: { $and: [ { $eq: [ "$arrayIndex", 0 ] }, { $eq: [ { $size: "$otherThanC" }, 0 ] } ] } , then: 1},
{ case: { $eq: [ "$arrayIndex", "$firstOtherThanCIndex" ] }, then: 1 }
],
default: 0
}
}
}
},
{
$group: {
_id: "$_id",
myArrayReversed: {
$push: {
value: "$reversed.value",
weight: "$weight"
}
}
}
},
{
$project: {
_id: 1,
myArray: { $reverseArray: "$myArrayReversed" }
}
}
])
Brief description of each pipeline stage:
We need to add two extra fields: $size of and array and second array with reversed items
Second and third steps are to find first (last) item not equal to C using $filter and $arrayElemAt which returns first matching index
Then we can $unwind our reversed array using special syntax which adds index of array when unwinding
That is the moment when we can calculate weight using $switch - simply setting 1 if array has one element or indexes are matching and zero otherwise
Then we just need to reshape the data: grouping back by _id and reversing the array