How to create an SQL View with SQLAlchemy?

Question

Is there a "Pythonic" way (I mean, no "pure SQL" query) to define an SQL view with SQLAlchemy?

Answer 1

Update: SQLAlchemy now has a great usage recipe here on this topic, which I recommend. It covers different SQL Alchemy versions up to the latest and has ORM integration (see comments below this answer and other answers). And if you look through the version history, you can also learn why using literal_binds is iffy (in a nutshell: binding parameters should be left to the database), but still arguably any other solution would make most users of the recipe not happy. I leave the below answer mostly for historical reasons.

Original answer: Creating a (read-only non-materialized) view is not supported out of the box as far as I know. But adding this functionality in SQLAlchemy 0.7 is straightforward (similar to the example I gave here). You just have to write a compiler extension CreateView. With this extension, you can then write (assuming that t is a table object with a column id)

createview = CreateView('viewname', t.select().where(t.c.id>5))
engine.execute(createview)

v = Table('viewname', metadata, autoload=True)
for r in engine.execute(v.select()):
    print r

Here is a working example:

from sqlalchemy import Table
from sqlalchemy.ext.compiler import compiles
from sqlalchemy.sql.expression import Executable, ClauseElement

class CreateView(Executable, ClauseElement):
    def __init__(self, name, select):
        self.name = name
        self.select = select

@compiles(CreateView)
def visit_create_view(element, compiler, **kw):
    return "CREATE VIEW %s AS %s" % (
         element.name,
         compiler.process(element.select, literal_binds=True)
         )

# test data
from sqlalchemy import MetaData, Column, Integer
from sqlalchemy.engine import create_engine
engine = create_engine('sqlite://')
metadata = MetaData(engine)
t = Table('t',
          metadata,
          Column('id', Integer, primary_key=True),
          Column('number', Integer))
t.create()
engine.execute(t.insert().values(id=1, number=3))
engine.execute(t.insert().values(id=9, number=-3))

# create view
createview = CreateView('viewname', t.select().where(t.c.id>5))
engine.execute(createview)

# reflect view and print result
v = Table('viewname', metadata, autoload=True)
for r in engine.execute(v.select()):
    print r

If you want, you can also specialize for a dialect, e.g.

@compiles(CreateView, 'sqlite')
def visit_create_view(element, compiler, **kw):
    return "CREATE VIEW IF NOT EXISTS %s AS %s" % (
         element.name,
         compiler.process(element.select, literal_binds=True)
         )

Answer 2

stephan's answer is a good one and covers most bases, but what left me unsatisfied was the lack of integration with the rest of SQLAlchemy (the ORM, automatic dropping etc.). After hours of experimenting and piecing together knowledge from all corners of the internet I came up with the following:

import sqlalchemy_views
from sqlalchemy import Table
from sqlalchemy.ext.compiler import compiles
from sqlalchemy.sql.ddl import DropTable


class View(Table):
    is_view = True


class CreateView(sqlalchemy_views.CreateView):
    def __init__(self, view):
        super().__init__(view.__view__, view.__definition__)


@compiles(DropTable, "postgresql")
def _compile_drop_table(element, compiler, **kwargs):
    if hasattr(element.element, 'is_view') and element.element.is_view:
        return compiler.visit_drop_view(element)

    # cascade seems necessary in case SQLA tries to drop 
    # the table a view depends on, before dropping the view
    return compiler.visit_drop_table(element) + ' CASCADE'

Note that I am utilizing the sqlalchemy_views package, just to simplify things.

Defining a view (e.g. globally like your Table models):

from sqlalchemy import MetaData, text, Text, Column


class SampleView:
    __view__ = View(
        'sample_view', MetaData(),
        Column('bar', Text, primary_key=True),
    )

    __definition__ = text('''select 'foo' as bar''')

# keeping track of your defined views makes things easier
views = [SampleView]

Mapping the views (enable ORM functionality):

Do when loading up your app, before any queries and after setting up the DB.

for view in views:
    if not hasattr(view, '_sa_class_manager'):
        orm.mapper(view, view.__view__)

Creating the views:

Do when initializing the database, e.g. after a create_all() call.

from sqlalchemy import orm


for view in views:
    db.engine.execute(CreateView(view))

How to query a view:

results = db.session.query(SomeModel, SampleView).join(
    SampleView,
    SomeModel.id == SampleView.some_model_id
).all()

This would return exactly what you expect (a list of objects that each has a SomeModel object and a SampleView object).

Dropping a view:

SampleView.__view__.drop(db.engine)

It will also automatically get dropped during a drop_all() call.

This is obviously a very hacky solution but in my eyes it is the best one and cleanest one out there at the moment. I have tested it these past few days and have not had any issues. I'm not sure how to add in relationships (ran into problems there) but it's not really necessary, as demonstrated above in the query.

If anyone has any input, finds any unexpected issues, or knows a better way to do things, please do leave a comment or let me know.

This was tested on SQLAlchemy 1.2.6 and Python 3.6.

Answer 3

These days there's a PyPI package for that: SQLAlchemy Views.

From it's PyPI Page:

>>> from sqlalchemy import Table, MetaData
>>> from sqlalchemy.sql import text
>>> from sqlalchemy_views import CreateView, DropView

>>> view = Table('my_view', metadata)
>>> definition = text("SELECT * FROM my_table")

>>> create_view = CreateView(view, definition, or_replace=True)
>>> print(str(create_view.compile()).strip())
CREATE OR REPLACE VIEW my_view AS SELECT * FROM my_table

However, you asked for a no "pure SQL" query, so you probably want the definition above to be created with SQLAlchemy query object.

Luckily, the text() in the example above makes it clear that the definition parameter to CreateView is such a query object. So something like this should work:

>>> from sqlalchemy import Table, Column, Integer, String, MetaData, ForeignKey
>>> from sqlalchemy.sql import select
>>> from sqlalchemy_views import CreateView, DropView

>>> metadata = MetaData()

>>> users = Table('users', metadata,
...     Column('id', Integer, primary_key=True),
...     Column('name', String),
...     Column('fullname', String),
... )

>>> addresses = Table('addresses', metadata,
...   Column('id', Integer, primary_key=True),
...   Column('user_id', None, ForeignKey('users.id')),
...   Column('email_address', String, nullable=False)
...  )

Here is the interesting bit:

>>> view = Table('my_view', metadata)
>>> definition = select([users, addresses]).where(
...     users.c.id == addresses.c.user_id
... )
>>> create_view = CreateView(view, definition, or_replace=True)
>>> print(str(create_view.compile()).strip())
CREATE OR REPLACE VIEW my_view AS SELECT users.id, users.name,
users.fullname, addresses.id, addresses.user_id, addresses.email_address 
FROM users, addresses 
WHERE users.id = addresses.user_id

Answer 4

SQLAlchemy-utils just added this functionality in 0.33.6 (available in pypi). It has views, materialized views, and it integrates with the ORM. It is not documented yet, but I am successfully using the views + ORM.

You can use their test as an example for both regular and materialized views using the ORM.

To create a view, once you install the package, use the following code from the test above as a base for your view:

class ArticleView(Base):
    __table__ = create_view(
        name='article_view',
        selectable=sa.select(
            [
                Article.id,
                Article.name,
                User.id.label('author_id'),
                User.name.label('author_name')
            ],
            from_obj=(
                Article.__table__
                    .join(User, Article.author_id == User.id)
            )
        ),
        metadata=Base.metadata
    )

Where Base is the declarative_base, sa is the SQLAlchemy package, and create_view is a function from sqlalchemy_utils.view.

Answer 5

Loosely based on https://github.com/sqlalchemy/sqlalchemy/wiki/Views

Complete executable example with sqlalchemy only, hope you don't spend hours just to make it run.

import sqlalchemy as sa
import sqlalchemy.schema
import sqlalchemy.ext.compiler


engine = sa.create_engine('postgresql://localhost/postgres')
meta = sa.MetaData()

Session = sa.orm.sessionmaker(bind=engine)
session = Session()


class Drop(sa.schema.DDLElement):
    def __init__(self, name, schema):
        self.name   = name
        self.schema = schema


class Create(sa.schema.DDLElement):
    def __init__(self, name, select, schema='public'):
        self.name   = name
        self.schema = schema
        self.select = select

        sa.event.listen(meta, 'after_create', self)
        sa.event.listen(meta, 'before_drop', Drop(name, schema))


@sa.ext.compiler.compiles(Create)
def createGen(element, compiler, **kwargs):
    return 'CREATE OR REPLACE VIEW {schema}."{name}" AS {select}'.format(
        name   = element.name,
        schema = element.schema,
        select = compiler.sql_compiler.process(
            element.select,
            literal_binds = True
        ),
    )


@sa.ext.compiler.compiles(Drop)
def dropGen(element, compiler, **kw):
    return 'DROP VIEW {schema}."{name}"'.format(
        name   = element.name,
        schema = element.schema,
    )


if __name__ == '__main__':
    view = Create(
        name   = 'myview',
        select = sa.select(sa.literal_column('1 AS col'))
    )
    meta.create_all(bind=engine, checkfirst=True)
    print(session.execute('SELECT * FROM myview').all())
    session.close()

Answer 6

I couldn't find an short and handy answer.

I don't need extra functionality of View (if any), so I simply treat a view as an ordinary table as other table definitions.

So basically I have a.py where defines all tables and views, sql related stuff, and main.py where I import those class from a.py and use them.

Here's what I add in a.py and works:

class A_View_From_Your_DataBase(Base):
    __tablename__ = 'View_Name'
    keyword = Column(String(100), nullable=False, primary_key=True)

Notably, you need to add the primary_key property even though there's no primary key in the view.

Answer 7

SQL View without pure SQL? You can create a class or function to implement a defined view.

function get_view(con):
  return Table.query.filter(Table.name==con.name).first()