Precondition

A precondition on a document, used for conditional operations.

JSON representation 
{

  // Union field condition_type can be only one of the following:
  "exists": boolean,
  "updateTime": string
  // End of list of possible types for union field condition_type.
}
Fields
Union field condition_type. The type of precondition. condition_type can be only one of the following:
exists

boolean

When set to true, the target document must exist. When set to false, the target document must not exist.
updateTime

string (Timestamp format)

When set, the target document must exist and have been last updated at that time. Timestamp must be microsecond aligned.

A timestamp in RFC3339 UTC "Zulu" format, with nanosecond resolution and up to nine fractional digits. Examples: "2014-10-02T15:01:23Z" and "2014-10-02T15:01:23.045123456Z".