Add ActiveValue::set_if_not_equals()

src/entity/active_model.rs

@@ -849,6 +849,43 @@ where
             None => ActiveValue::NotSet,
         };
     }
+
+    /// `Set(value)`, except when [`self.is_unchanged()`][ActiveValue#method.is_unchanged]
+    /// and `value` equals the current [Unchanged][ActiveValue::Unchanged] value.
+    ///
+    /// This is useful when you have an [Unchanged][ActiveValue::Unchanged] value from the database,
+    /// then update it using this method,
+    /// and then use [`.is_unchanged()`][ActiveValue#method.is_unchanged] to see whether it has *actually* changed.
+    ///
+    /// The same nice effect applies to the entire `ActiveModel`.
+    /// You can now meaningfully use [ActiveModelTrait::is_changed][ActiveModelTrait#method.is_changed]
+    /// to see whether are any changes that need to be saved to the database.
+    ///
+    /// ## Examples
+    ///
+    /// ```
+    /// # use sea_orm::ActiveValue;
+    /// #
+    /// let mut value = ActiveValue::Unchanged("old");
+    ///
+    /// // This wouldn't be the case if we used plain `value = Set("old");`
+    /// value.set_if_not_equals("old");
+    /// assert!(value.is_unchanged());
+    ///
+    /// // Only when we change the actual `&str` value, it becomes `Set`
+    /// value.set_if_not_equals("new");
+    /// assert_eq!(value.is_unchanged(), false);
+    /// assert_eq!(value, ActiveValue::Set("new"));
+    /// ```
+    pub fn set_if_not_equals(&mut self, value: V)
+    where
+        V: PartialEq,
+    {
+        match self {
+            ActiveValue::Unchanged(current) if &value == current => {}
+            _ => *self = ActiveValue::Set(value),
+        }
+    }
 }
 
 impl<V> std::convert::AsRef<V> for ActiveValue<V>