Computed Columns in Azure Mobile Services

[yourmobileservicesschema].[GetAvgRatingForPicture].sql

CREATE FUNCTION [yourmobileservicesschema].[GetAvgRatingForPicture](@pictureId bigint)
RETURNS int
AS
BEGIN
    DECLARE @r decimal(3,2)
    select @r = AVG(rating) from PictureRatings where picture_id = @pictureId
    RETURN @r
END

[yourmobileservicesschema].[Pictures].sql

CREATE TABLE [yourmobileservicesschema].[Pictures] (
    [id]                      BIGINT             IDENTITY (1, 1) NOT NULL,
    [average_rating]          AS                 ([yourmobileservicesschema].[GetAvgRatingForPicture]([id])),
    PRIMARY KEY CLUSTERED ([id] ASC)
);

pictures.insert.js

function insert(item, user, request) {
    delete item.average_rating;
    request.execute();
}