Implement map expressions

[katcharov]

JAVA-4817

objectToArray / arrayToObject (which correspond to Java's map.entrySet, and reduce-entries-to-map) require a Map type (as well as an Entry type)

[jyemin]

I don't see a way to generate something like{ $objectToArray: "$instock" } which is the most common usage of maps that I can think of. Seems like we need either a new method in Expressions or a way to convert a DocumentExpression to a MapExpression.

[katcharov]

Right you are - getMap is missing, and needs to be added. This would be doc.getMap("instock").entrySet(). Passing test:

    DocumentExpression doc = of(Document.parse("{ instock: { warehouse1: 2500, warehouse2: 500 } }"));
    assertExpression(
            Arrays.asList(
                    Document.parse("{'k': 'warehouse1', 'v': 2500}"),
                    Document.parse("{'k': 'warehouse2', 'v': 500}")),
            doc.getMap("instock").entrySet(),
            "{'$objectToArray': {'$getField': {'input': {'$literal': "
                    + "{'instock': {'warehouse1': 2500, 'warehouse2': 500}}}, 'field': 'instock'}}}");

Let's get #1050 and #1052 resolved, and I will be able to merge those and #1053 in, and then rebase to fix this?

[katcharov]

Added getMap and isMapOr in aae819e.

Incidentally, the fact that maps are equal to documents introduces an unavoidable way of converting between them (using isMapOr/isDocumentOr).

[katcharov]

Rebased, no conflicts.