Selecting the first item of an ARRAY with PostgreSQL/SqlAlchemy

Question

Trying to move some queries I run daily into an automated script. I have one in Postgres like the below:

SELECT regexp_split_to_array(col1, "|")[1] AS item, COUNT(*) AS itemcount FROM Tabel1 GROUP BY item ORDER BY itemcount

In SqlAlchemy I have this:

session.query((func.regexp_split_to_array(model.table1.col1, "|")[1]).label("item"), func.count().label("itemcount")).group_by("item").order_by("itemcount")

Python can't "get_item" since it's not actually a collection. I've looked through the docs and can't seem to find something that would let me do this without running raw SQL using execute (which I can do and works, but was looking for a solution for next time).

univerio · Accepted Answer · 2017-09-23 05:54:14Z

7

SQLAlchemy does support indexing with [...]. If you declare a type of a column that you have to be of type postgresql.ARRAY, then it works:

table2 = Table("table2", meta, Column("col1", postgresql.ARRAY(String)))
q = session.query(table2.c.col1[1])
print(q.statement.compile(dialect=postgresql.dialect()))
# SELECT table2.col1[%(col1_1)s] AS anon_1 
# FROM table2

The reason why your code doesn't work is that SQLAlchemy does not know that func.regexp_split_to_array(...) returns an array, since func.foo produces a generic function for convenience. To make it work, we need to make sure SQLAlchemy knows the return type of the function, by specifying the type_ parameter:

q = session.query(func.regexp_split_to_array(table1.c.col1, "|", type_=postgresql.ARRAY(String))[1].label("item"))
print(q.statement.compile(dialect=postgresql.dialect()))
# SELECT (regexp_split_to_array(table1.col1, %(regexp_split_to_array_1)s))[%(regexp_split_to_array_2)s] AS item 
# FROM table1

answered Sep 23, 2017 at 5:54

univerio

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1 Comment

DR913 Over a year ago

Requires sqlalchemy.types for String (which I didn't have imported for reasons that not good), but otherwise works like a charm. Thanks!

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