Add some consistency checks to SQL model

backend/sql/version_1.sql

@@ -263,7 +263,16 @@ CREATE TABLE [ShoppingEntry] (
 
     CHECK (
         length([name]) <= 255 AND
-        ([is_checked] = TRUE OR [is_checked] = FALSE)
+        ([is_checked] = TRUE OR [is_checked] = FALSE) AND
+        (
+            [recipe_scheduled_id] IS NULL OR
+            (
+                [name] = '' AND
+                [quantity_value] IS NULL AND
+                [quantity_unit] = '' AND
+                [servings] IS NULL
+            )
+        )
     ),
 
     FOREIGN KEY([user_id]) REFERENCES [User]([id]) ON DELETE CASCADE,
@@ -284,20 +293,21 @@ BEGIN
         [name] = OLD.[name],
         [quantity_value] = OLD.[quantity_value],
         [quantity_unit] = OLD.[quantity_unit],
-        [servings] = (
+        [servings] = COALESCE((
         	SELECT [servings]
         	FROM [Recipe]
         	INNER JOIN [Group] ON [Group].[recipe_id] = [Recipe].[id]
         	INNER JOIN [Step] ON [Step].[group_id] = [Group].[id]
         	WHERE [Step].[id] = OLD.[step_id]
-    	)
+            LIMIT 1 -- In case a step is owned by more than one recipe (should never happen).
+    	), 4)
     WHERE [ingredient_id] = OLD.[id];
 END;
 
 CREATE TABLE [Settings] (
     [name] TEXT NOT NULL PRIMARY KEY,
 
-    -- Value can by anything that can be read from a text by
+    -- Value can be anything that can be read from a text by
     -- implementing the trait 'std::str::FromStr'.
     [value] TEXT NOT NULL
 ) STRICT;