Fix word name collision, add two new user-related features

This commit changes the primary key of words to a serial number. That
way, two words with the same normalized value will not collide with
one another.

It also adds two new tables in the database:
- Users following languages
- Users learning words

The former can represent two stages of learning a word:
- Either the user is currently learning it
- Or they consider they know it and don’t need to work on it anymore

These two new tables now have their API query available through the
GraphQL API.

This commit also fixes the issue of word-related tables and types not
being dropped when resetting the database.

migrations/2023-01-03-134434_create_word/down.sql

@@ -1 +1,7 @@
--- This file should undo anything in `up.sql`
+-- This file should undo anything in `up.sql`
+DROP TABLE WordRelation;
+DROP TABLE WordLearning;
+DROP TABLE Words;
+DROP TYPE WordLearningStatus;
+DROP TYPE WordRelationship;
+DROP TYPE PartOfSpeech;

migrations/2023-01-03-134434_create_word/up.sql

@@ -1,12 +1,14 @@
 -- Your SQL goes here
 CREATE TYPE PartOfSpeech as ENUM ('ADJ', 'ADP', 'ADV', 'AUX', 'CCONJ', 'DET', 'INTJ', 'NOUN', 'NUM', 'PART', 'PRON', 'PROPN', 'PUNCT', 'SCONJ', 'SYM', 'VERB', 'X');
 CREATE TYPE WordRelationship as ENUM('def', 'related');
+CREATE TYPE WordLearningStatus as ENUM('learning', 'learned');
 
 CREATE TABLE Words (
-  norm VARCHAR(255) PRIMARY KEY, -- normalized word
+  id UUID DEFAULT uuid_generate_v4 () PRIMARY KEY,
+  norm VARCHAR(255) NOT NULL, -- normalized word, generally in latin alphabet
   native VARCHAR(255),
-  lemma VARCHAR(255)
-    REFERENCES Words(norm)
+  lemma UUID
+    REFERENCES Words(id)
     ON UPDATE CASCADE
     ON DELETE SET NULL,
   language UUID
@@ -26,15 +28,30 @@ CREATE TABLE Words (
 
 CREATE TABLE WordRelation (
   id SERIAL PRIMARY KEY,
-  wordsource VARCHAR(255)
-    REFERENCES Words(norm)
+  wordsource UUID
+    REFERENCES Words(id)
     ON UPDATE CASCADE
     ON DELETE CASCADE
     NOT NULL,
-  wordtarget VARCHAR(255)
-    REFERENCES Words(norm)
+  wordtarget UUID
+    REFERENCES Words(id)
     ON UPDATE CASCADE
     ON DELETE CASCADE
     NOT NULL,
   relationship WordRelationship NOT NULL
 );
+
+CREATE TABLE WordLearning (
+  id SERIAL PRIMARY KEY,
+  word UUID
+    REFERENCES Words(id)
+    ON UPDATE CASCADE
+    ON DELETE CASCADE
+    NOT NULL,
+  userid VARCHAR(31)
+    REFERENCES Users(id)
+    ON UPDATE CASCADE
+    ON DELETE CASCADE
+    NOT NULL,
+  status WordLearningStatus DEFAULT 'learning' NOT NULL
+);