sqlalchemy exists（） - 如何避免多餘From

Question

exists（）包含另一個exists（）會導致額外的From子句。

model.session.query(Table1.id).\ 
    filter(~ exists().\ 
     where(Table2.table1_id==Table1.id).\ 
     where(~ exists().\ 
       where(Table3.contract_id==Table2.contract_id).\ 
       where(Table3.session_id==Table1.session_id)) 
     ) 

這是產生：

SELECT table1.id AS table1_id FROM table1 
WHERE NOT (EXISTS (SELECT * FROM table2 
      WHERE table2.table1_id = table1.id 
      AND NOT (EXISTS (SELECT * FROM table3, table1 
        WHERE table3.contract_id = table2.contract_id 
        AND table3.session_id = table1.session_id)))) 

這裏，「FROM表1」中的最後一個「存在」是不是必需的，因爲表1是已經在最上面的查詢。我如何強制sqlalchemy不添加這個額外的「FROM table1」？

我真正想要的是：

SELECT table1.id AS table1_id FROM table1 
WHERE NOT (EXISTS (SELECT * FROM table2 
      WHERE table2.table1_id = table1.id 
      AND NOT (EXISTS (SELECT * FROM table3 
        WHERE table3.contract_id = table2.contract_id 
        AND table3.session_id = table1.session_id)))) 

我不知道如何實現這一目標。 有人可以幫我嗎？ 使用SQLAlchemy 0.7.9。

Answer 1

q = (session.query(Table1.id) 
    .filter(~exists(
     select([Table2.id]) 
     .where(Table2.table1_id == Table1.id) 
     .where(~exists(
      # changing exists to be implicit enables the 'important' below 
      select([Table3.id]) 
      .where(Table3.contract_id == Table2.contract_id) 
      .where(Table3.session_id == Table1.session_id) 
      # this is important 
      .correlate(Table1) 
      .correlate(Table2) 
      )) 
    )))