System parameter common to all googleapis for filtering fields

query parameter	header
fields	X-Goog-FieldMask

FieldMask contains a list of paths

Basics

• Specify multiple field paths by seperating with a comma
• A field path can be indexed with a . or /
• Sub-paths can be specified within ()
• Can specify all fields with *

Example

• e.g. {a:[a,b,c]}
• to only get {a:[c]} you would use a/c or a.c or a(c)
• To include both b and c to get {a:[b,c]} you can use a(b,c)

Example 2

• e.g. {a:[a,b,c],d:[e,f]}
• a(b,c),d.f will return {a:[b,c],d:[f]}
• a(*) or a will return {a:[a,b,c]}

Remarks

• Default is * (i.e. X-Goog-FieldMask: *) (return all). This may be configured differently across APIs.
• Specifying a field that does not exist or no longer exists will return an error
• I don't think there is a way to exclude a field and get all others

Further information

• https://cloud.google.com/apis/docs/system-parameters
• https://developers.google.com/drive/api/guides/fields-parameter
• https://github.com/protocolbuffers/protobuf/blob/main/src/google/protobuf/field_mask.proto