class TestMetadata(Base):
    id = Column(Integer,primary_key=True)
    name = Column(String)
    Metadata_item = Column(JSONB)

按照SQLAlchemy documentation of JSONB(搜索Path索引操作示例)：

expr = TestMetadata.Metadata_item[("nested_field","a simple text")]
q = (session.query(TestMetadata.id,expr.label("deep_value"))
     .filter(expr != None)
     .all())

哪个应生成以下sql：

SELECT  testMetadata.id AS testMetadata_id,testMetadata.Metadata_item #> %(Metadata_item_1)s AS deep_value
FROM    testMetadata
WHERE  (testMetadata.Metadata_item #> %(Metadata_item_1)s) IS NOT NULL
-- @params: {'Metadata_item_1': u'{nested_field,a simple text}'}