src/Table.php
public function getJoinField(Table $table): ?Field
    {
        $field = $table->getForeignKey();

        return $this->fields[$field] ?? null;
    }
public function getJoinField(Table $table): ?Field
    {
        $field = $table->getForeignKey();

        return $this->fields[$field] ?? null;
    }
Returns the foreign key.