Refactor [Tag] table: remove lang column. Remove [Tag] when no more recipe uses it.

backend/sql/version_1.sql

@@ -111,15 +111,17 @@ CREATE TABLE [Image] (
 CREATE TABLE [Tag] (
     [id] INTEGER PRIMARY KEY,
     [name] TEXT NOT NULL,
-    [lang] TEXT NOT NULL DEFAULT 'en',
+
+    -- Not needed, the lang is defined by the recipes linked to it
+    -- (more than one language can be associaded to a tag).
+    -- [lang] TEXT NOT NULL DEFAULT 'en',
 
     CHECK (
-        length([name]) <= 31 AND
-        length([lang]) = 2
+        length([name]) <= 31
     )
 ) STRICT;
 
-CREATE UNIQUE INDEX [Tag_name_lang_index] ON [Tag]([name], [lang]);
+CREATE UNIQUE INDEX [Tag_name_index] ON [Tag]([name]);
 
 CREATE TABLE [RecipeTag] (
     [id] INTEGER PRIMARY KEY,
@@ -133,6 +135,18 @@ CREATE TABLE [RecipeTag] (
     FOREIGN KEY([tag_id]) REFERENCES [Tag]([id]) ON DELETE CASCADE
 ) STRICT;
 
+CREATE INDEX [RecipeTag_tag_id_index] ON [RecipeTag]([tag_id]);
+
+-- Delete all tags without references.
+CREATE TRIGGER [RecipeTag_trigger_delete]
+AFTER DELETE
+ON [RecipeTag]
+BEGIN
+    DELETE FROM [Tag] WHERE
+        [id] = OLD.[tag_id] AND
+        (SELECT COUNT(*) = 0 FROM [RecipeTag] WHERE [tag_id] = OLD.[tag_id]);
+END;
+
 CREATE TABLE [Group] (
     [id] INTEGER PRIMARY KEY,