Field Mask Support

gRPC APIs commonly use Field Masks to support projections.

Rejoiner can automatically populate a FieldMask from the GraphQL query. This allows the fields selected in a GraphQL query to propagate from all the way to the backend.

For example, fields selected in a GraphQL query can be passed to the Backend API in the Field Mask parameter, which can then be used as part of a projection when querying the database.

Field Masks

FieldMask represents a set of symbolic field paths, for example:

   paths: "f.a"
   paths: "f.b.d"

Here f represents a field in some root message, a and b fields in the message found in f, and d a field found in the message in f.b.

Field masks are used to specify a subset of fields that should be returned by a get operation or modified by an update operation.

Field Masks in Projections

When used in the context of a projection, a response message or sub-message is filtered by the API to only contain those fields as specified in the mask. For example, if the mask in the previous example is applied to a response message as follows:

   f {
     a : 22
     b {
       d : 1
       x : 2
     }
     y : 13
   }
   z: 8

The result will not contain specific values for fields x,y and z (their value will be set to the default, and omitted in proto text output):

   f {
     a : 22
     b {
       d : 1
     }
   }