Flask Marshmallow/SqlAlchemy: Serializing many-to-many relationships
In your BookSchema, you'll want to add a Nested authors field like you had (but commented out) but you'll want to specify that it will be a list by using the many keyword argument.
class BookSchema(ma.ModelSchema): # A list of author objects authors = ma.Nested(AuthorSchema, many=True) class Meta: model = Book