```python
engine = create_engine(engineUrl, convert_unicode=True, poolclass=NullPool)

def jsonExtract(item, key):
    j = json.loads(item)
    value = j.get(key)
    return value

@sqlalchemy.event.listens_for(engine, "begin")
def do_begin(conn):
    if conn.dialect.name == 'sqlite':
        conn.connection.create_function('jsonExtract', 2, jsonExtract)

class SqliteJsonElement(FunctionElement):
    type = types.Text()
    name = "JsonElement"

    @property
    def astext(self):
        return self


@compiles(SqliteJsonElement)
def compileSqliteJsonElement(element, compiler, **kw):
    return "jsonExtract(%s)" % compiler.process(element.clauses)


class SqliteJson(TypeDecorator):
    impl = types.Text

    def process_bind_param(self, value, dialect):
        return json.dumps(value)

    def process_result_value(self, value, dialect):
        return json.loads(value)

    class comparator_factory(sqltypes.Concatenable.Comparator):
        def __getitem__(self, other):
            """Get the value at a given key."""
            return SqliteJsonElement(self.expr, other)


class SqliteCompatiblePostgresOptimizedJson(TypeDecorator):
    # See: http://docs.sqlalchemy.org/en/latest/core/custom_types.html
    impl = None

    def load_dialect_impl(self, dialect):
        dialectTypeMap = {
            'sqlite': SqliteJson,
            'postgresql': JSONB
        }

        self.impl = dialect.type_descriptor(dialectTypeMap[dialect.name])
        return self.impl

    def process_bind_param(self, value, dialect):
        return value

    def process_result_value(self, value, dialect):
        return value

    @property
    def comparator_factory(self):
        """express comparison behavior in terms of the base type"""
        return self.impl.comparator_factory
```