Foreign key enforcement in SQLite

Foreign keys are disabled by default in SQLite3. It would be nice to have an optional enable_foreign_keys boolean argument to the SqliteDatabase constructor. It would have two effects. Suppose db = SqliteDatabase(filename, enable_foreign_keys=True). Then

1. db.foreign_keys is True; and
2. db.create_tables would issue

PRAGMA foreign_keys = ON;

Issuing that PRAGMA in the middle of a multi-statement transaction has no effect and swallows the error. It is only possible to turn on foreign_keys in SQLite versions greater than 3.6.19 when SQLite is compiled with SQLITE_OMIT_TRIGGER and SQLITE_OMIT_FOREIGN_KEY turned off. The versions of SQLite compiled into Python versions 2.7.9 and 3.4.3 on Windows appear to support foreign_keys.

[coleifer]

SqliteDatabase can now be instantiated with pragmas, so:

db = SqliteDatabase('filename.db', pragmas=(('foreign_keys', 'on'),))

[coleifer]

Specifying AUTOINCREMENT on id fields seems like a good idea.

This has performance implications. Hence, we use the default SQLite behavior but allow you to use auto-increment if you really want that. http://docs.peewee-orm.com/en/latest/peewee/playhouse.html#PrimaryKeyAutoIncrementField