dbDelta using Foreign key not working on update [duplicate]

From the Codex:

Note that the dbDelta function is rather picky, however. For instance: […]

• You must not use any apostrophes or backticks around field names.

$sql = "CREATE TABLE ".TEST_TABLE." (
    id INT( 11 ) NOT NULL AUTO_INCREMENT,
    title VARCHAR( 100 ) NOT NULL,
    description TEXT DEFAULT NULL,
    location_id INT( 11 ) NOT NULL,
    PRIMARY KEY  (id),
    FOREIGN KEY  (location_id) REFERENCES ".TEST2_TABLE." (id)
);";