How can I generate a DB, that fits my room scheme?

Question

I have a database with quite a lot of Entities and I want to preload data from a file, on first creation of the database. For that the scheme of Room needs to fit the scheme of the database file. Since converting the json scheme by hand to SQLite statements is very error-prone ( I would need to copy paste every single of the statements and exchange the variable names) I am looking for a possibility to automatically generate a database from the scheme, that I then just need to fill with the data. However apparently there´s no information if that is possible or even how to do so, out in the internet. It´s my first time working with SQLite (normally I use MySQL) and also the first time I see a database scheme in json. (Since standard MariaDB export options always just export the CREATE TABLE statements.) Is there a way? Or does Room provide anyway to actually get the create table statements as a proper text, not split up in tons of JSON arrays? I followed the guide on Android Developer Guidelines to get the json-scheme, so I have that file already. For those, who do not know it´s structure, it looks like this:

{
  "formatVersion": 1,
  "database": {
    "version": 1,
    "identityHash": "someAwesomeHash",
    "entities": [
      {
        "tableName": "Articles",
        "createSql": "CREATE TABLE IF NOT EXISTS `${TABLE_NAME}` (`id` INTEGER NOT NULL, `germanArticle` TEXT NOT NULL, `frenchArticle` TEXT, PRIMARY KEY(`id`))",
        "fields": [
          {
            "fieldPath": "id",
            "columnName": "id",
            "affinity": "INTEGER",
            "notNull": true
          },
          {
            "fieldPath": "germanArticle",
            "columnName": "germanArticle",
            "affinity": "TEXT",
            "notNull": true
          },
          {
            "fieldPath": "frenchArticle",
            "columnName": "frenchArticle",
            "affinity": "TEXT",
            "notNull": false
          }
        ],
        "primaryKey": {
          "columnNames": [
            "id"
          ],
          "autoGenerate": false
        },
        "indices": [
          {
            "name": "index_Articles_germanArticle",
            "unique": true,
            "columnNames": [
              "germanArticle"
            ],
            "createSql": "CREATE UNIQUE INDEX IF NOT EXISTS `index_Articles_germanArticle` ON `${TABLE_NAME}` (`germanArticle`)"
          },
          {
            "name": "index_Articles_frenchArticle",
            "unique": true,
            "columnNames": [
              "frenchArticle"
            ],
            "createSql": "CREATE UNIQUE INDEX IF NOT EXISTS `index_Articles_frenchArticle` ON `${TABLE_NAME}` (`frenchArticle`)"
          }
        ],
        "foreignKeys": []
      },
...

Note: My question was not, how to create the Room DB out of the scheme. To receive the scheme, I already had to create all the Entities and the database. But how to get the structure Room creates as SQL to prepopulate my Database. However, I think the answer is a really nice explanation, and in fact I found the SQL-Statements I was searching for in the generated Java-file, which was an awesome hint. ;)

Answer 1

Is there a way? Or does Room provide anyway to actually get the create table statements as a proper text, not split up in tons of JSON arrays?

You cannot simply provide the CREATE SQL for Room, what you need to do is to generate the java/Kotlin classes (Entities) from the JSON and then add those classes to the project.

• native SQLite (i.e. not using Room) would be a different matter as it could do done at runtime.

The way Room works is that the database is generated from the classes annotated with @Entity (at compile time).

The Entity/classes have to exist for the compile to correctly generate the code that it generates.

Furthermore the Entity(ies) have to be incorporated/included into a class for the Database, that being annotated with @Database (this class is typically abstract).

Yet furthermore to access the database tables you have abstract classes or interfaces for the SQL each being annotated with @Dao and again these require the Entity classes as the SQL is checked at compile time.

e.g. the JSON you provided would equate to something like :-

@Entity(
        indices = {
                @Index(value = "germanArticle", name = "index_Articles_germanArticle", unique = true),
                @Index(value = "frenchArticle", name = "index_Articles_frenchArticle", unique = true)
        }
        , primaryKeys = {"id"}
)
public class Articles {
    //@PrimaryKey // Could use this as an alternative
    long id;
    @NonNull
    String germanArticle;
    String frenchArticle;
}

• so your process would have to convert the JSON to create the above and which could then be copied into the project.

You would then need a Class for the database which could be for example :-

@Database(entities = {Articles.class},version = 1)
abstract class MyDatabase extends RoomDatabase {
}

• Note that Dao classes would be added to body of the above along the lines of :-

    abstract MyDaoClass getDao();
  

Or does Room provide anyway to actually get the create table statements as a proper text, not split up in tons of JSON arrays?

Yes it does ....

At this stage if you compile it generates java (MyDatabase_Impl for the above i.e. the name of the Database class suffixed with _Impl). However as there are no Dao classes/interfaces. The database would unusable from a Room perspective (and thus wouldn't even get created).

part of the code generated would be :-

  @Override
  public void createAllTables(SupportSQLiteDatabase _db) {
    _db.execSQL("CREATE TABLE IF NOT EXISTS `Articles` (`id` INTEGER NOT NULL, `germanArticle` TEXT NOT NULL, `frenchArticle` TEXT, PRIMARY KEY(`id`))");
    _db.execSQL("CREATE UNIQUE INDEX IF NOT EXISTS `index_Articles_germanArticle` ON `Articles` (`germanArticle`)");
    _db.execSQL("CREATE UNIQUE INDEX IF NOT EXISTS `index_Articles_frenchArticle` ON `Articles` (`frenchArticle`)");
    _db.execSQL("CREATE TABLE IF NOT EXISTS room_master_table (id INTEGER PRIMARY KEY,identity_hash TEXT)");
    _db.execSQL("INSERT OR REPLACE INTO room_master_table (id,identity_hash) VALUES(42, 'f7294cddfc3c1bc56a99e772f0c5b9bb')");
  }

• As you can see the Articles table and the two indices are created, the room_master_table is used for validation checking.