Implement $addFields stage dsl

[username1103]

resolve #79

---

Currently, the $sum operator is implemented as follows.

spring-data-mongodb-kotlin-dsl/core/src/main/kotlin/com/github/inflab/spring/data/mongodb/core/aggregation/expression/AggregationExpressionDsl.kt

Lines 176 to 178 in 661cc3e

	fun sum() = AggregationExpression {
	Document("\$sum", 1)
	}

But We need to use it as follows.

db.scores.aggregate( [
   {
     $addFields: {
       totalHomework: { $sum: "$homework" } ,
       totalQuiz: { $sum: "$quiz" }
     }
   },
   {
     $addFields: { totalScore:
       { $add: [ "$totalHomework", "$totalQuiz", "$extraCredit" ] } }
   }
] )

[jbl428]

please add the @param docs for all methods.

[jbl428]

There is withValueOf method for field path.
But test case is failed if used it.
Bad luck 😢

[username1103]

Yes. So use withValue 🥲

[jbl428]

You can use @Document annotation, then it will be more convenient.

[username1103]

Even when using the @Document annotation, it doesn't recognize 'id' as '_id'. I think it might be possible to achieve this by modifying the toFieldName according to the content of the following document.

https://docs.spring.io/spring-data/mongodb/docs/2.1.3.RELEASE/reference/html/#mongo-template.id-handling

[jbl428]

I think we can use @id annotation :) (same as remaining)

[username1103]

Even when using the @Id anntation, it doesn't recognize 'id' as '_id' in this case.

    fun overwriteAnExistingFieldWithField(): AggregationResults<Fruit> {
        val aggregation = aggregation {
            addFields {
                Fruit::id set Fruit::item
                Fruit::item set "fruit"
            }
        }

        return mongoTemplate.aggregate<Fruit>(aggregation, FRUITS)
    }

[jbl428]

I understand, then you can update toFieldName to read @id annotation as document.

[username1103]

Update toFieldName to check @Id annotation.

spring data monogo also has _id handling rule that a property or field without an annotation but named id maps to the _id field. docs

Shouldn't we add this rule?

[jbl428]

Yes, and there are some hardcoded "_id" literal in example repositories.
you can update them too.

[jbl428]

The example that use $sum operator will be added when it is implemented completely.
I think that there might be some changes after this PR is merged.

LTGM 👍