feat: add list of aliases to note entity

One of the aliases can be primary for each note, but all can be used to get information from the apis.

Signed-off-by: Philip Molares <philip.molares@udo.edu>

src/notes/alias.entity.ts

@@ -0,0 +1,63 @@
+/*
+ * SPDX-FileCopyrightText: 2021 The HedgeDoc developers (see AUTHORS file)
+ *
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+import {
+  Column,
+  Entity,
+  ManyToOne,
+  PrimaryGeneratedColumn,
+  Unique,
+} from 'typeorm';
+
+import { Note } from './note.entity';
+import { PrimaryValueTransformer } from './primary.value-transformer';
+
+@Entity()
+@Unique('Only one primary alias per note', ['note', 'primary'])
+export class Alias {
+  @PrimaryGeneratedColumn('uuid')
+  id: string;
+
+  /**
+   * the actual alias
+   */
+  @Column({
+    nullable: false,
+    unique: true,
+  })
+  name: string;
+
+  /**
+   * Is this alias the primary alias, by which people access the note?
+   */
+  @Column({
+    /*
+      Because of the @Unique at the top of this entity, this field must be saved as null instead of false in the DB.
+      If a non-primary alias would be saved with `primary: false` it would only be possible to have one non-primary and one primary alias.
+      But a nullable field does not have such problems.
+      This way the DB keeps track that one note really only has one primary alias.
+      */
+    comment:
+      'This field tells you if this is the primary alias of the note. If this field is null, that means this alias is not primary.',
+    nullable: true,
+    transformer: new PrimaryValueTransformer(),
+  })
+  primary: boolean;
+
+  @ManyToOne((_) => Note, (note) => note.aliases, {
+    onDelete: 'CASCADE', // This deletes the Alias, when the associated Note is deleted
+  })
+  note: Note;
+
+  // eslint-disable-next-line @typescript-eslint/no-empty-function
+  private constructor() {}
+
+  static create(name: string, primary = false): Alias {
+    const alias = new Alias();
+    alias.name = name;
+    alias.primary = primary;
+    return alias;
+  }
+}