Writing a query in SQLAlchemy to count occurrences and store IDs - postgresql

Question

I'm working with a postgres db using SQLAlchemy.
I have a table like this
class Author(Base):
__tablename__ = "Author"
id = Column(BIGINT, primary_key=True)
name = Column(Unicode)
and I want to identify all homonymous authors and save their id in a list.
For example if in the database there are 2 authors named "John" and 3 named "Jack", with ID respectively 11, 22, 33, 44 a 55, I want my query to return
[("John", [11,22]), ("Jack", [33,44,55])]
For now I've been able to write
[x for x in db_session.query(
func.count(Author.name),
Author.name
).group_by(Author.name) if x[0]>1]
but this just gives me back occurrences
[(2,"John"),(3,"Jack")]
Thank you very much for the help!

Answer 1

The way to do this in SQL would be to use PostgreSQL's array_agg function to group the ids into an array:
SELECT
name,
array_agg(id) AS ids
FROM
my_table
GROUP BY
name
HAVING
count(name) > 1;
The array_agg function collects the ids for each name, and the HAVING clause excludes those with only a single row. The output of the query would look like this:
name │ ids
═══════╪════════════════════
Alice │ {2,4,9,10,16}
Bob │ {1,6,11,12,13}
Carol │ {3,5,7,8,14,15,17}
Translated into SQLAlchemy, the query would look like this:
import sqlalchemy as sa
...
q = (
db_session.query(Author.name, sa.func.array_agg(Author.id).label('ids'))
.group_by(Author.name)
.having(sa.func.count(Author.name) > 1)
)
Calling q.all() will return a list of (name, [ids]) tuples like this:
[
('Alice', [2, 4, 9, 10, 16]),
('Bob', [1, 6, 11, 12, 13]),
('Carol', [3, 5, 7, 8, 14, 15, 17]),
]
In SQLAlchemy 1.4/2.0-style syntax equivalent would be:
with Session() as s:
q = (
sa.select(Author.name, sa.func.array_agg(Author.id).label('ids'))
.group_by(Author.name)
.having(sa.func.count(Author.name) > 1)
)
res = s.execute(q)