Using PostgreSQL array to store many-to-many relationship

Question

Suppose we have a PostgreSQL database with two tables A, B.

table A columns: id, name
table B columns: id, name, array_a

The column array_a in table B contains a variable length array of ids from table A. In SQLAlchemy we have two classes that model those tables, say class A and B.

The following works fine to get all the objects A that are referenced in an object B:

session.query(A).join(B, A.id == func.any(B.array_a)).filter(B.id == <id>).all()

How can we create a relationship in B referencing the objects A corresponding to the array? Tried column comparators using the func.any above but it complains that ANY(array_a) is not a column in the model. Specifying the primaryjoin conditions as above doesn't seem to cut it either.

SingleNegationElimination · Accepted Answer · 2012-03-15 23:59:24Z

13

This anti-pattern is called "Jaywalking"; and PostgreSQL's powerful type system makes it very tempting. you should be using another table:

CREATE TABLE table_a (
    id SERIAL PRIMARY KEY,
    name VARCHAR
);

CREATE TABLE table_b (
    id SERIAL PRIMARY KEY,
    name VARCHAR
);

CREATE TABLE a_b (
    a_id INTEGER PRIMARY KEY REFERENCES table_a(id),
    b_id INTEGER PRIMARY KEY REFERENCES table_b(id)
)

Which is mapped:

from sqlalchemy import *
from sqlalchemy.dialects import postgresql
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import *

Base = declarative_base()

a_b_table = Table("a_b", Base.metadata,
    Column("a_id", Integer, ForeignKey("table_a.id"), primary_key=True),
    Column("b_id", Integer, ForeignKey("table_b.id"), primary_key=True))

class A(Base):
    __tablename__ = "table_a"
    id = Column(Integer, primary_key=True)
    name = Column(String)

class B(Base):
    __tablename__ = "table_b"
    id = Column(Integer, primary_key=True)
    name = Column(String)
    a_set = relationship(A, secondary=a_b_table, backref="b_set")

example:

>>> print Query(A).filter(A.b_set.any(B.name == "foo"))
SELECT table_a.id AS table_a_id, table_a.name AS table_a_name 
FROM table_a 
WHERE EXISTS (SELECT 1 
FROM a_b, table_b 
WHERE table_a.id = a_b.a_id AND table_b.id = a_b.b_id AND table_b.name = :name_1)

If you are stuck with the ARRAY column, your best bet is to use an alternate selectable that "looks" like a proper association table.

from sqlalchemy import *
from sqlalchemy.dialects import postgresql
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import *

Base = declarative_base()


class A(Base):
    __tablename__ = "table_a"
    id = Column(Integer, primary_key=True)
    name = Column(String)

class B(Base):
    __tablename__ = "table_b"
    id = Column(Integer, primary_key=True)
    name = Column(String)
    array_a = Column(postgresql.ARRAY(Integer))

a_b_selectable = select([func.unnest(B.array_a).label("a_id"),
                         B.id.label("b_id")]).alias()

A.b_set = relationship(B, secondary=a_b_selectable,
                          primaryjoin=A.id == a_b_selectable.c.a_id,
                          secondaryjoin=a_b_selectable.c.b_id == B.id,
                          viewonly=True,)

B.a_set = relationship(A, secondary=a_b_selectable,
                          primaryjoin=A.id == a_b_selectable.c.a_id,
                          secondaryjoin=a_b_selectable.c.b_id == B.id,
                          viewonly=True)

which gives you:

>>> print Query(A).filter(A.b_set.any(B.name == "foo"))
SELECT table_a.id AS table_a_id, table_a.name AS table_a_name 
FROM table_a 
WHERE EXISTS (SELECT 1 
FROM (SELECT unnest(table_b.array_a) AS a_id, table_b.id AS b_id 
FROM table_b) AS anon_1, table_b 
WHERE table_a.id = anon_1.a_id AND anon_1.b_id = table_b.id AND table_b.name = :name_1)

And obviously, since there's no real table there, viewonly=True is neccesary and you can't get the nice, dynamic objecty goodness you would if you had avoided jaywalking.

edited Mar 15, 2012 at 23:59

answered Mar 15, 2012 at 23:43

SingleNegationElimination

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5 Comments

ThiefMaster Over a year ago

I think relation is a deprecated alias for relationships since a few versions.

TR. Over a year ago

The selectable does what we need, yes.

Kiran Jonnalagadda Over a year ago

I'm not sure it's jaywalking when PostgreSQL arrays are queryable and have a well defined type. It seems more an SQLAlchemy limitation with how it handles PostgreSQL arrays.

Kiran Jonnalagadda Over a year ago

Thanks for the SQL Antipatterns reference. I picked up the book today and it's fabulous.

SingleNegationElimination Over a year ago

@Kiran Jonnalagadda: there's nothing wrong with using the array type; it's certainly very useful, even via sqlalchemy's types. The limitation is not because the Array type is poor, it isn't; but rather, enforcing data integrity through composite types is both difficult to express and difficult to get right. With a simple, single cell-per-referent arrangement, the consistency constraint can be expressed as just references footable(foocolumn), that's what makes it jaywalking.

KR32 · Accepted Answer · 2021-07-06 18:23:30Z

5

Or else simply you can join explicitly like below:

class A(Base):
    __tablename__ = "table_a"
    id = Column(Integer, primary_key=True)
    name = Column(String)

class B(Base):
    __tablename__ = "table_b"
    id = Column(Integer, primary_key=True)
    name = Column(String)
    array_a = Column(postgresql.ARRAY(Integer))

    a_ids= relationship('A',primaryjoin='A.id == any_(foreign(B.array_a))',uselist=True)

answered Jul 6, 2021 at 18:23

KR32

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