Why I need to specify $switch default statement while it is optional? - mongodb

Question

I want to update the documents with a condition. I tried $cond, but it needs an expression for false case. And I don't want to update anything if it is false. Below is the sample of the document:
{
"_id" : ObjectId("5bc29e0d0fc2c40a9d628afe"),
"BasicInfo" : {
"RepNo" : "AE179",
"CompanyName" : "First Bancshares Inc",
"IRSNo" : "640862173",
"CIKNo" : "0000947559",
"Name" : "Ordinary Shares",
"Ticker" : "FBMS",
"CUSIP" : "318916103",
"ISIN" : "US3189161033",
"RIC" : "FBMS.O",
"SEDOL" : "2184300",
"DisplayRIC" : "FBMS.OQ",
"InstrumentPI" : "10552665",
"QuotePI" : "26300255",
"Exchange" : "NASDAQ"
},
"Annual" : {
"Date" : ISODate("2017-12-31T00:00:00.000Z"),
"INC" : {
"SIIB" : {
"Description" : "Interest Income, Bank",
"Value" : 66.06941
},
"STIE" : {
"Description" : "Total Interest Expense",
"Value" : 6.90925
},
"ENII" : {
"Description" : "Net Interest Income",
"Value" : 59.16016
Then, I tried to use $switch since the documentation said default statement is optional.
And I have written the following code:
db.getCollection('FinancialStatement').aggregate([
{"$unwind":"$Annual"},
{"$addFields":{"Annual.Price":
{"$switch":{
branches:[
{
case: {
"$and":[
{"$eq":["$_id", ObjectId("5bc29e0d0fc2c40a9d628afe")]},
{"$eq":["$Annual.Date", ISODate("2017-12-31 00:00:00.000Z")]}
]
},
then: 1000}
],
default: -2000
}
}
}
}
]
)
It basically add a new field called annual.price if objectID and date requirements are met. However, if I omitt the default statement, the program returns an error saying :
Assert: command failed: {
"ok" : 0,
"errmsg" : "$switch could not find a matching branch for an input, and no default was specified.",
"code" : 40066,
"codeName" : "Location40066"
}

Answer 1

From docs on usage of default
Optional. The path to take if no branch case expression evaluates to
true.
Although optional, if default is unspecified and no branch case
evaluates to true, $switch returns an error.
Use $$REMOVE in 3.6.
Something like
Using $cond
{"$addFields":{
"Annual.Price":{
"$cond":[
{
"$and":[
{"$eq":["$_id",ObjectId("5bc29e0d0fc2c40a9d628afe")]},
{"$eq":["$Annual.Date",ISODate("2017-12-31 00:00:00.000Z")]}
]
},
1000,
"$$REMOVE"
]
}
}}
Using $switch
{"$addFields":{
"Annual.Price":{
"$switch":{
"branches":[
{
"case":{
"$and":[
{"$eq":["$_id",ObjectId("5bc29e0d0fc2c40a9d628afe")]},
{"$eq":["$Annual.Date",ISODate("2017-12-31 00:00:00.000Z")]}
]
},
"then":1000
}
],
"default":"$$REMOVE"
}
}
}}