Macro to is automatic conversion from symbol/variable BsonDocument to specified Type. This doesn't support dynamic values of array, only support homogeneous value-type array. It can convert only when the field is exported or using special pragma bsonExport to enable its conversion without exporting the field itself to other modules.

For custom type conversion, user can provide the proc, func or converter named with pattern of "of" & Type.name. For example, the Type is SimpleObj so the proc name is ofSimpleObj. The complete example is shown below:

import anonimongo/core/bson
type
  Embedtion = object
    embedfield*: int
    embedstat*: string
    wasProcInvoked: bool
  SimpleEmbedObject = object
    intfield*: int
    strfield*: string
    embed*: Embedtion

proc ofEmbedtion(b: BsonBase): Embedtion =
  let embed = b.ofEmbed
  result.embedfield = embed["embedfield"]
  result.embedstat = embed["embedstat"]
  result.wasProcInvoked = true

let bsimple = bson({
  intfield: 42,
  strfield: "that's 42",
  embed: {
    embedfield: 42,
    embedstat: "42",
  },
})
let simple = bsimple.to SimpleEmbedObject
doAssert simple.intfield == 42
doAssert simple.strfield == "that's 42"
doAssert simple.embed.embedfield == 42
doAssert simple.embed.embedstat == "42"
doAssert simple.embed.wasProcInvoked

Note that the ofType isn't checked for the outer most of Type because if user wants implement the specifics conversion, the user just can simply call it immediately without resorting to macro to.