sqlalchemy using INTERSECT and UNNEST

Question

I'm trying to translate a raw SQL to sqlalchemy core/orm but I'm having some difficulties. Here is the SQL:

SELECT 
    (SELECT UNNEST(MyTable.my_array_column) 
     INTERSECT 
     SELECT UNNEST(ARRAY['VAL1', 'VAL2']::varchar[])) AS matched
FROM 
    MyTable
WHERE 
    my_array_column && ARRAY['VAL1', 'VAL2']::varchar[];

The following query, gives me a FROM clause which I don't need in my nested SELECT:

matched = select([func.unnest(MyTable.my_array_column)]).intersect(select([func.unnest('VAL1', 'VAL2')]))

# SELECT unnest(MyTable.my_array_colum) AS unnest_1 
# FROM MyTable INTERSECT SELECT unnest(%(unnest_3)s, %(unnest_4)s) AS unnest_2

How can I tell the select to not include the FROM clause? Note that func.unnest() only accepts a column. So I cannot use func.unnest('my_array_column').

univerio · Accepted Answer · 2016-11-02 19:37:52Z

Referring to a table of an enclosing query in a subquery is the process of correlation, which SQLAlchemy attempts to do automatically. In this case, it doesn't quite work, I believe, because your INTERSECT query is a "selectable", not a scalar value, which SQLAlchemy attempts to put in the FROM list instead of the SELECT list.

The solution is twofold. We need to make SQLAlchemy put the INTERSECT query in the SELECT list by applying a label, and make it correlate MyTable correctly:

select([
    select([func.unnest(MyTable.my_array_column)]).correlate(MyTable)
    .intersect(select([func.unnest('VAL1', 'VAL2')]))
    .label("matched")
]).select_from(MyTable)

# SELECT (SELECT unnest("MyTable".my_array_column) AS unnest_1 INTERSECT SELECT unnest(%(unnest_3)s, %(unnest_4)s) AS unnest_2) AS matched 
# FROM "MyTable"

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