Simplify AggregationExpression usage in Aggregation.project [DATAMONGO-2311]

[spring-projects-issues]

Christoph Strobl opened DATAMONGO-2311 and commented

Currently one has to call project().and(ctx -> ....)

by adding the following this could be reduced to project(ctx -> ...)

public ProjectionOperationBuilder and(AggregationExpression expression) {
	return new ProjectionOperationBuilder(expression, this, null);
}

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Referenced from: pull request #769

[spring-projects-issues]

Mark Paluch commented

We should make sure that method called on the actual ProjectionOperation consider the entire projection:

assertThat(project(ctx -> new Document("foo", 1)).andExclude("bar", "foo")
      .toDocument(Aggregation.DEFAULT_CONTEXT))
            .isEqualTo(new Document("$project", new Document("foo", 0).append("bar", 0))); 

Right now, the first document is exposed as _id field

[spring-projects-issues]

Christoph Strobl commented

currently a Projection needs to expose a field which must not be the case if it's a document that can be dynamically created.
The solution in the PR needs to hold a projection instance in order to be chain able