Remove handling orphaned foreign keys

[morozov]

The functionality being removed was deprecated in #5758.

[morozov]

This test is being removed as invalid. Consider instrumenting it with the following code:

$platform = new MySQLPlatform();

echo '-- Schema', PHP_EOL;
foreach ($oldSchema->toSql($platform) as $statement) {
    echo $statement, ';', PHP_EOL;
}

echo '-- Diff', PHP_EOL;
foreach ($schemaDiff->toSql($platform) as $statement) {
    echo $statement, ';', PHP_EOL;
}

If run on 4.0.x, it produces the following statements (the PRIMARY KEY clauses are not defined by the test schema and were added manually for the sake of correctness):

-- Schema
CREATE TABLE table_a (id INT NOT NULL PRIMARY KEY);
CREATE TABLE table_b (id INT NOT NULL PRIMARY KEY);
CREATE TABLE table_c (id INT NOT NULL, table_a_id INT NOT NULL, table_b_id INT NOT NULL, INDEX IDX_7365E39D71081393 (table_a_id), INDEX IDX_7365E39D63BDBC7D (table_b_id));
ALTER TABLE table_c ADD CONSTRAINT FK_7365E39D71081393 FOREIGN KEY (table_a_id) REFERENCES table_a (id);
ALTER TABLE table_c ADD CONSTRAINT FK_7365E39D63BDBC7D FOREIGN KEY (table_b_id) REFERENCES table_b (id);

-- Diff
ALTER TABLE table_a DROP FOREIGN KEY FK_7365E39D71081393;
DROP TABLE table_a;
ALTER TABLE table_c DROP FOREIGN KEY FK_7365E39D63BDBC7D;
DROP INDEX IDX_7365E39D71081393 ON table_c;
DROP INDEX IDX_7365E39D63BDBC7D ON table_c;
ALTER TABLE table_c DROP table_a_id, DROP table_b_id;

The assertion in question passes only because the DROP FOREIGN KEY FK_7365E39D71081393 statement is mistakenly executed on table_a although it should be executed on table_c.

With the functionality of handling orphaned foreign keys removed, the test starts failing because now the SQL looks like this:

-- Schema
CREATE TABLE table_a (id INT NOT NULL PRIMARY KEY);
CREATE TABLE table_b (id INT NOT NULL PRIMARY KEY);
CREATE TABLE table_c (id INT NOT NULL, table_a_id INT NOT NULL, table_b_id INT NOT NULL, INDEX IDX_7365E39D71081393 (table_a_id), INDEX IDX_7365E39D63BDBC7D (table_b_id));
ALTER TABLE table_c ADD CONSTRAINT FK_7365E39D71081393 FOREIGN KEY (table_a_id) REFERENCES table_a (id);
ALTER TABLE table_c ADD CONSTRAINT FK_7365E39D63BDBC7D FOREIGN KEY (table_b_id) REFERENCES table_b (id);

-- Diff
DROP TABLE table_a;
ALTER TABLE table_c DROP FOREIGN KEY FK_7365E39D71081393;
ALTER TABLE table_c DROP FOREIGN KEY FK_7365E39D63BDBC7D;
DROP INDEX IDX_7365E39D71081393 ON table_c;
DROP INDEX IDX_7365E39D63BDBC7D ON table_c;
ALTER TABLE table_c DROP table_a_id, DROP table_b_id;

Both constraints are dropped on table_c (which is correct) but this is not what the assertion expects.

Note that the resulting SQL is still invalid because table_a cannot be dropped while it's still referenced by a foreign key. The functionality of dropping multiple tables should be reworked accordingly (see the API introduced in #5416) but it seems to be out of the scope of this change since it didn't work properly even before this change.

[greg0ire]

A test that was passing by accident… must have been quite confusing, thanks for the detailed explanation!