Implement general update operator $rename

[tatu-at-datastax]

Another general-purpose array operator to implement is $rename. Its semantics are similar to $unset followed by $set (if that was allowed), so that:

1. If target path exists, the value is overwritten
2. If source path does not exist, operation does nothing (but does not fail)

Format is simple:

{ "$rename" : { "field1" : "newName1", .... , "fieldN", "newNamen" } }

Following error modes exist:

1. _id either as source or target: cannot modify immutable property "_id"
2. Same source and target (must differ)
3. Attempt to move source to incompatible path

The last case occurs, for example when an attempt is madeto set a property on Array value: that is, target path points to something that is not possible to add: for example with

{ "x" : 3, "array" : [ 1 ] }

attempt to apply operation

{ "$rename" : { "x" : "array.x" }  }

would fail. Same is true for paths "through" atomic fields; only (sub)documents have properties.

[vkarpov15]

$rename isn't necessarily an array operator. I'd also put $rename as low priority, it is rarely used in my experience.

[tatu-at-datastax]

Thank you @vkarpov15 that was a copy-paste mistake (I know it's not array-specific).

I am creating issues based on @amorton's list; this is lower priority than my current work on dotted paths for sure.