professional-programming/antipatterns/sqlalchemy-antipatterns.md

<!-- START doctoc generated TOC please keep comment here to allow auto update -->
<!-- DON'T EDIT THIS SECTION, INSTEAD RE-RUN doctoc TO UPDATE -->
## Table of Contents

- [SQLAlchemy Anti-Patterns](#sqlalchemy-anti-patterns)
  - [Abusing lazily loaded relationships](#abusing-lazily-loaded-relationships)
  - [Explicit session passing](#explicit-session-passing)
  - [Implicit transaction handling](#implicit-transaction-handling)
  - [Loading the full object when checking for object existence](#loading-the-full-object-when-checking-for-object-existence)
  - [Using identity as comparator](#using-identity-as-comparator)
  - [Returning `None` instead of raising a `NoResultFound` exception](#returning-none-instead-of-raising-a-noresultfound-exception)

<!-- END doctoc generated TOC please keep comment here to allow auto update -->

# SQLAlchemy Anti-Patterns

This is a list of what I consider [SQLAlchemy](http://www.sqlalchemy.org/)
anti-patterns.

## Abusing lazily loaded relationships

Bad:

```python
class Customer(Base):

    @property
    def has_valid_toast(self):
        """Return True if customer has at least one valid toast."""
        return any(toast.kind == 'brioche' for toast in self.toaster.toasts)
```

This suffers from severe performance inefficiencies:

- The toaster will be loaded, as well as its toast. This involves creating and
  issuing the SQL query, waiting for the database to return, and instantiating
  all those objects.
- `has_valid_toast` does not actually care about those objects. It just returns
  a boolean.

A better way would be to issue a SQL `EXISTS` query so that the database
handles this check and only returns a boolean.

Good:

```python
class Customer(Base):

    @property
    def has_valid_toast(self):
        """Return True if customer has at least one valid toast."""
        query = (session.query(Toaster)
                 .join(Toast)
                 .with_parent(self)
                 .filter(Toast.kind == 'brioche'))
        return session.query(query.exists()).scalar()
```

This query might not always be the fastest if those relationships are small,
and eagerly loaded.

## Explicit session passing

TODO

Bad:

```python
def toaster_exists(toaster_id, session):
    ...
```

## Implicit transaction handling

TODO

## Loading the full object when checking for object existence

Bad:

```python
def toaster_exists(toaster_id):
    return bool(session.query(Toaster).filter_by(id=toaster_id).first())
```

This is inefficient because it:

- Queries all the columns from the database (including any eagerly loaded joins)
- Instantiates and maps all data on the Toaster model

The database query would look something like this. You can see that all columns
are selected to be loaded by the ORM.

```sql
SELECT toasters.id AS toasters_id, toasters.name AS toasters_name,
toasters.color AS toasters_color
FROM toasters
WHERE toasters.id = 1
 LIMIT 1 OFFSET 0
```

And then it just checks if the result is truthy.

Here's a better way to do it:

```python
def toaster_exists(toaster_id):
    query = session.query(Toaster).filter_by(id=toaster_id)
    return session.query(query.exists()).scalar()
```

In this case, we just ask the database about whether a record exists with this
id. This is obviously much more efficient.

```sql
SELECT EXISTS (SELECT 1
FROM toasters
WHERE toasters.id = 1) AS anon_1
```

## Using identity as comparator

Bad:

```python
toasters = session.query(Toaster).filter(Toaster.deleted_at is None).all()
```

Unfortunately this won't work at all. This query will return all toasters,
including the one that were deleted.

The way sqlalchemy works is that it overrides the magic comparison methods
(`__eq__`, `__lt__`, etc.). All comparison methods can be overrode except the
identity operator (`is`) which checks for objects identity.

What this means is that expression `Toaster.deleted_at is None` will be
immediately evaluated by the Python interpreter, and since (presumably)
`Toaster.deleted_at` is a `sqlalchemy.orm.attributes.InstrumentedAttribute`,
it's not `None` and thus it's equivalent to doing:

```python
toasters = session.query(Toaster).filter(True).all()
```

Which obviously renders the filter inoperable, and will return all records.

There's two ways to fix it:

```python
toasters = session.query(Toaster).filter(Toaster.deleted_at == None).all()
```

Here we use the equality operator, which Python allows overriding. Behind the
scene, Python calls `Toaster.deleted_at.__eq__(None)`, which gives SQLAlchemy
the opportunity to return a comparator that when coerced to a string, will
evaluate to `deleted_at is NULL`.

Most linter will issue a warning for equality comparison against `None`, so you
can also do (this is my preferred solution):

```python
toasters = session.query(Toaster).filter(Toaster.deleted_at.is_(None)).all()
```

See docs for
[is\_](http://docs.sqlalchemy.org/en/rel_1_0/core/sqlelement.html#sqlalchemy.sql.operators.ColumnOperators.is_).

## Returning `None` instead of raising a `NoResultFound` exception

See [Returning nothing instead of raising NotFound exception](https://github.com/charlax/antipatterns/blob/master/code-antipatterns.md#returning-nothing-instead-of-raising-notfound-exception).