SQLAlchemy, Array_agg, And Matching An Input List

I am attempting to use SQLAlchemy more fully, rather than just falling back to pure SQL at the first sign of distress. In this case, I have a table in a Postgres database (9.5) wh

Solution 1:

I think your solution would produce indeterminate results, because the rows within a group are in unspecified order, and so the comparison between the array aggregate and given array may produce true or false based on that:

[local]:5432 u@sopython*=> select group_id
[local] u@sopython- > from multi_groups 
[local] u@sopython- > group by group_id
[local] u@sopython- > having array_agg((atom_id, sequence)) = ARRAY[(1,3),(2,2),(3,1)];
 group_id 
----------
        2
(1 row)

[local]:5432 u@sopython*=> update multi_groups set atom_id = atom_id where atom_id = 2;
UPDATE 2
[local]:5432 u@sopython*=> select group_id                                             
from multi_groups 
group by group_id
having array_agg((atom_id, sequence)) = ARRAY[(1,3),(2,2),(3,1)];
 group_id 
----------
(0 rows)

You could apply an ordering to both, or try something entirely different: instead of array comparison you could use relational division.

In order to divide you have to form a temporary relation from your list of Entity records. Again, there are many ways to approach that. Here's one using unnested arrays:

In [112]: vtm = select([
     ...:     func.unnest(postgresql.array([
     ...:         getattr(e, f) for e in values_to_match
     ...:     ])).label(f)
     ...:     for f in Entity._fields
     ...: ]).alias()

And another using a union:

In [114]: vtm = union_all(*[
     ...:     select([literal(e.atom_id).label('atom_id'),
     ...:             literal(e.sequence).label('sequence')])
     ...:     for e in values_to_match
     ...: ]).alias()

A temporary table would do as well.

With the new relation at hand you want to find the answer to "find those multi_groups for which no entity exists that is not in the group". It's a horrible sentence, but makes sense:

In [117]: mg = aliased(MultiColumnGroups)

In [119]: session.query(MultiColumnGroups.group_id).\
     ...:     filter(~exists().
     ...:         select_from(vtm).
     ...:         where(~exists().
     ...:             where(MultiColumnGroups.group_id == mg.group_id).
     ...:             where(tuple_(vtm.c.atom_id, vtm.c.sequence) ==
     ...:                   tuple_(mg.atom_id, mg.sequence)).
     ...:             correlate_except(mg))).\
     ...:     distinct().\
     ...:     all()
     ...: 
Out[119]: [(2)]

---

On the other hand you could also just select the intersection of groups with the given entities:

In [19]: gs = intersect(*[
    ...:     session.query(MultiColumnGroups.group_id).
    ...:         filter(MultiColumnGroups.atom_id == vtm.atom_id,
    ...:                MultiColumnGroups.sequence == vtm.sequence)
    ...:     for vtm in values_to_match
    ...: ])

In [20]: session.execute(gs).fetchall()
Out[20]: [(2,)]

---

The error

ProgrammingError: (psycopg2.ProgrammingError) operator does not exist: record[] = integer[]
LINE 3: ...gg((multi_groups.atom_id, multi_groups.sequence)) = ARRAY[AR...
                                                             ^
HINT:  No operator matches the given name and argument type(s). You might need to add explicit type casts.
 [SQL: 'SELECT multi_groups.group_id AS multi_groups_group_id \nFROM multi_groups GROUP BY multi_groups.group_id \nHAVING array_agg((multi_groups.atom_id, multi_groups.sequence)) = %(array_agg_1)s'] [parameters: {'array_agg_1': [[1, 3], [2, 2], [3, 1]]}] (Background on this error at: http://sqlalche.me/e/f405)

is a result of how your values_to_match is first converted to a list of lists (for reasons unknown) and then converted to an array by your DB-API driver. It results in an array of array of integer, not an array of record (int, int). Using a raw DB-API connection and cursor, passing a list of tuples works as you'd expect.

In SQLAlchemy if you wrap the list values_to_match with sqlalchemy.dialects.postgresql.array(), it works as you meant it to work, though remember that the results are indeterminate.