Union partitioned table and view, then query using _PARTITIONTIME not _pt

I have a partitioned table called table1 and I need to query it as below:

SELECT * FROM table1
WHERE DATE(_PARTITIONTIME) = "2021-04-20"
AND id>50


Then I realised there are some missing data in table1 and I have to union with table2 (which is a view) to have complete data so :

WITH
unioned AS(
SELECT
id, type, _partitiontime
FROM
`table1`
WHERE
_PARTITIONTIME > "2021-04-17"
UNION ALL
SELECT
id, type, _pt as _partitiontime,
FROM
`table2`
WHERE
_pt > "2021-04-17")

SELECT * FROM unioned
WHERE _partitiontime = "2021-04-20"
AND id>50


so I get Invalid field name "_partitiontime". error. If I write my query as below then it will work:

WITH
unioned AS(
SELECT
id, type, _partitiontime as _pt
FROM
`table1`
WHERE
_PARTITIONTIME > "2021-04-17"
UNION ALL
SELECT
id, type, _pt,
FROM
`table2`
WHERE
_pt > "2021-04-17")

SELECT * FROM unioned
WHERE _pt = "2021-04-20"
AND id>50


but because there are lots of nested queries which are using the above query so I prefer to query the unioned table using _partitiontime not _pt otherwise I have to change lots of codes. Is there any way that the below query works:

SELECT * FROM unioned
WHERE _partitiontime = "2021-04-20"
AND id>50

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