Address review comments

This commit adds documentation, a changelog entry and a test to the new
feature

CHANGELOG.md

@@ -15,6 +15,7 @@ Increasing the minimal supported Rust version will always be coupled at least wi
 * Added `limit()` and `offset()` DSL to combination clauses such as `UNION`
 * Fixed `#[derive(Identifiable)]` ignoring attribute `#[diesel(serialize_as)]` on primary keys
 * Added embedded struct support for `AsChangeset` via `#[diesel(embed)]`
+* Added a `#[diesel(skip_update)]` attribute for the `AsChangeset` derive to skip updating a field present in the struct
 * Support for libsqlite3-sys 0.30.0
 * Add support for built-in PostgreSQL range operators and functions
 * Support for postgres multirange type

diesel_derives/src/lib.rs

@@ -111,6 +111,8 @@ mod valid_grouping;
 ///    the actual field type.
 /// * `#[diesel(treat_none_as_null = true/false)]`, overrides the container-level
 ///   `treat_none_as_null` attribute for the current field.
+/// * `#[diesel(skip_update)]`, skips updating this field. Useful for working with
+///    generated columns.
 #[cfg_attr(
     all(not(feature = "without-deprecated"), feature = "with-deprecated"),
     proc_macro_derive(

diesel_derives/tests/as_changeset.rs

@@ -891,3 +891,40 @@ fn embedded_struct() {
     let actual = users::table.order(users::id).load(connection);
     assert_eq!(Ok(expected), actual);
 }
+
+#[test]
+fn skip_update() {
+    let connection = &mut connection_with_sean_and_tess_in_users_table();
+
+    #[derive(AsChangeset)]
+    #[diesel(table_name = users)]
+    #[diesel(primary_key(name))]
+    #[allow(dead_code)]
+    struct UserForm<'a> {
+        #[diesel(skip_update)]
+        name: &'a str,
+        hair_color: &'a str,
+        r#type: &'a str,
+        #[diesel(skip_update)]
+        non_existing: i32,
+    }
+
+    diesel::update(users::table.find(1))
+        .set(UserForm {
+            name: "John",
+            hair_color: "green",
+            r#type: "admin",
+            non_existing: 42,
+        })
+        .execute(connection)
+        .unwrap();
+
+    let res = users::table
+        .find(1)
+        .first::<(i32, String, Option<String>, Option<String>)>(connection)
+        .unwrap();
+
+    assert_eq!(res.1, "Sean");
+    assert_eq!(res.2, Some("green".into()));
+    assert_eq!(res.3, Some("admin".into()));
+}