ReactiveStringBasedAggregation computes pipeline stages with domainClass instead of return type

Currently the #computePipelineStage(String, ConvertingParameterAccessor, ParameterBindingDocumentCodec) method in the ReactiveStringBasedAggregation class maps the pipeline operation sources to the domain class, resulting in fields in a $project stage being incorrectly being resolved as _id as its name in the expected return type (Bar) -> namely identifier

Consider the following repository with an aggretion query, and 2 data models;

@Repository
interface FooRepository extends ReactiveMongoRepository<Foo, String> {

    @Aggregation(pipeline = {
            "{ $match : { _id : '?0' } }",
            "{ $replaceRoot : { newRoot : '$bar' } }",
            "{ $project : { identifier : 1 } }"
    })
    Mono<Bar> findBarBy(String identifier);
}

@Getter
@Setter
@Document
class Foo {

    @Id private String identifier;
    private Bar bar;
}

@Getter
@Setter
@Document
class Bar {

    private String identifier;
}

Executing the aggregation results in an instance of Bar , however the identifier is null , due the fact that the ReactiveStringBasedAggregation maps the pipeline stages to the domain class and identifier is being transformed to _id

Added a sample project -> https://github.com/dnijssen/mongodb-aggregation

Start in ReactiveStringBasedAggregation at #computePipelineStage(String, ConvertingParameterAccessor, ParameterBindingDocumentCodec) and compare how pipeline sources are mapped to the domain class versus the aggregation return type. Use the linked sample project to reproduce the Foo-to-Bar query, then verify that the $project identifier is retained as identifier rather than resolved to _id.