SqlSoup — SQLAlchemy 0.7.1 Documentation

# SQLAlchemy 0.7.1 Documentation

Search:
Version: 0.7.1 Last Updated: 06/05/2011 13:13:59

# SqlSoup

## Introduction

SqlSoup provides a convenient way to access existing database tables without having to declare table or mapper classes ahead of time. It is built on top of the SQLAlchemy ORM and provides a super-minimalistic interface to an existing database.

SqlSoup effectively provides a coarse grained, alternative interface to working with the SQLAlchemy ORM, providing a “self configuring” interface for extremely rudimental operations. It’s somewhat akin to a “super novice mode” version of the ORM. While SqlSoup can be very handy, users are strongly encouraged to use the full ORM for non-trivial applications.

Suppose we have a database with users, books, and loans tables (corresponding to the PyWebOff dataset, if you’re curious).

Creating a SqlSoup gateway is just like creating an SQLAlchemy engine:

    >>> from sqlalchemy.ext.sqlsoup import SqlSoup
    >>> db = SqlSoup('sqlite:///:memory:')

or, you can re-use an existing engine:

    >>> db = SqlSoup(engine)

You can optionally specify a schema within the database for your SqlSoup:

    >>> db.schema = myschemaname

## Loading objects

Loading objects is as easy as this:

    >>> users = db.users.all()
    >>> users.sort()
    >>> users
    [
        MappedUsers(name=u'Joe Student',email=u'student@example.edu',
                password=u'student',classname=None,admin=0),
        MappedUsers(name=u'Bhargan Basepair',email=u'basepair@example.edu',
                password=u'basepair',classname=None,admin=1)
    ]

Of course, letting the database do the sort is better:

    >>> db.users.order_by(db.users.name).all()
    [
        MappedUsers(name=u'Bhargan Basepair',email=u'basepair@example.edu',
            password=u'basepair',classname=None,admin=1),
        MappedUsers(name=u'Joe Student',email=u'student@example.edu',
            password=u'student',classname=None,admin=0)
    ]

Field access is intuitive:

    >>> users[0].email
    u'student@example.edu'

Of course, you don’t want to load all users very often. Let’s add a WHERE clause. Let’s also switch the order_by to DESC while we’re at it:

    >>> from sqlalchemy import or_, and_, desc
    >>> where = or_(db.users.name=='Bhargan Basepair', db.users.email=='student@example.edu')
    >>> db.users.filter(where).order_by(desc(db.users.name)).all()
    [
        MappedUsers(name=u'Joe Student',email=u'student@example.edu',
            password=u'student',classname=None,admin=0),
        MappedUsers(name=u'Bhargan Basepair',email=u'basepair@example.edu',
            password=u'basepair',classname=None,admin=1)
    ]

You can also use .first() (to retrieve only the first object from a query) or .one() (like .first when you expect exactly one user – it will raise an exception if more were returned):

    >>> db.users.filter(db.users.name=='Bhargan Basepair').one()
    MappedUsers(name=u'Bhargan Basepair',email=u'basepair@example.edu',
            password=u'basepair',classname=None,admin=1)

Since name is the primary key, this is equivalent to

    >>> db.users.get('Bhargan Basepair')
    MappedUsers(name=u'Bhargan Basepair',email=u'basepair@example.edu',
        password=u'basepair',classname=None,admin=1)

This is also equivalent to

    >>> db.users.filter_by(name='Bhargan Basepair').one()
    MappedUsers(name=u'Bhargan Basepair',email=u'basepair@example.edu',
        password=u'basepair',classname=None,admin=1)

filter_by is like filter, but takes kwargs instead of full clause expressions. This makes it more concise for simple queries like this, but you can’t do complex queries like the or_ above or non-equality based comparisons this way.

### Full query documentation

Get, filter, filter_by, order_by, limit, and the rest of the query methods are explained in detail in Querying.

## Modifying objects

Modifying objects is intuitive:

    >>> user = _
    >>> user.email = 'basepair+nospam@example.edu'
    >>> db.commit()

(SqlSoup leverages the sophisticated SQLAlchemy unit-of-work code, so multiple updates to a single object will be turned into a single `UPDATE` statement when you commit.)

To finish covering the basics, let’s insert a new loan, then delete it:

    >>> book_id = db.books.filter_by(title='Regional Variation in Moss').first().id
    >>> db.loans.insert(book_id=book_id, user_name=user.name)
    MappedLoans(book_id=2,user_name=u'Bhargan Basepair',loan_date=None)

    >>> loan = db.loans.filter_by(book_id=2, user_name='Bhargan Basepair').one()
    >>> db.delete(loan)
    >>> db.commit()

You can also delete rows that have not been loaded as objects. Let’s do our insert/delete cycle once more, this time using the loans table’s delete method. (For SQLAlchemy experts: note that no flush() call is required since this delete acts at the SQL level, not at the Mapper level.) The same where-clause construction rules apply here as to the select methods:

    >>> db.loans.insert(book_id=book_id, user_name=user.name)
    MappedLoans(book_id=2,user_name=u'Bhargan Basepair',loan_date=None)
    >>> db.loans.delete(db.loans.book_id==2)

You can similarly update multiple rows at once. This will change the book_id to 1 in all loans whose book_id is 2:

    >>> db.loans.update(db.loans.book_id==2, book_id=1)
    >>> db.loans.filter_by(book_id=1).all()
    [MappedLoans(book_id=1,user_name=u'Joe Student',
        loan_date=datetime.datetime(2006, 7, 12, 0, 0))]

## Joins

Occasionally, you will want to pull out a lot of data from related tables all at once. In this situation, it is far more efficient to have the database perform the necessary join. (Here we do not have a lot of data but hopefully the concept is still clear.) SQLAlchemy is smart enough to recognize that loans has a foreign key to users, and uses that as the join condition automatically:

    >>> join1 = db.join(db.users, db.loans, isouter=True)
    >>> join1.filter_by(name='Joe Student').all()
    [
        MappedJoin(name=u'Joe Student',email=u'student@example.edu',
            password=u'student',classname=None,admin=0,book_id=1,
            user_name=u'Joe Student',loan_date=datetime.datetime(2006, 7, 12, 0, 0))
    ]

If you’re unfortunate enough to be using MySQL with the default MyISAM storage engine, you’ll have to specify the join condition manually, since MyISAM does not store foreign keys. Here’s the same join again, with the join condition explicitly specified:

    >>> db.join(db.users, db.loans, db.users.name==db.loans.user_name, isouter=True)
    <class 'sqlalchemy.ext.sqlsoup.MappedJoin'>

You can compose arbitrarily complex joins by combining Join objects with tables or other joins. Here we combine our first join with the books table:

    >>> join2 = db.join(join1, db.books)
    >>> join2.all()
    [
        MappedJoin(name=u'Joe Student',email=u'student@example.edu',
            password=u'student',classname=None,admin=0,book_id=1,
            user_name=u'Joe Student',loan_date=datetime.datetime(2006, 7, 12, 0, 0),
            id=1,title=u'Mustards I Have Known',published_year=u'1989',
            authors=u'Jones')
    ]

If you join tables that have an identical column name, wrap your join with with_labels, to disambiguate columns with their table name (.c is short for .columns):

    >>> db.with_labels(join1).c.keys()
    [u'users_name', u'users_email', u'users_password',
        u'users_classname', u'users_admin', u'loans_book_id',
        u'loans_user_name', u'loans_loan_date']

You can also join directly to a labeled object:

    >>> labeled_loans = db.with_labels(db.loans)
    >>> db.join(db.users, labeled_loans, isouter=True).c.keys()
    [u'name', u'email', u'password', u'classname',
        u'admin', u'loans_book_id', u'loans_user_name', u'loans_loan_date']

## Relationships

You can define relationships on SqlSoup classes:

    >>> db.users.relate('loans', db.loans)

These can then be used like a normal SA property:

    >>> db.users.get('Joe Student').loans
    [MappedLoans(book_id=1,user_name=u'Joe Student',
                    loan_date=datetime.datetime(2006, 7, 12, 0, 0))]

    >>> db.users.filter(~db.users.loans.any()).all()
    [MappedUsers(name=u'Bhargan Basepair',
            email='basepair+nospam@example.edu',
            password=u'basepair',classname=None,admin=1)]

relate can take any options that the relationship function accepts in normal mapper definition:

    >>> del db._cache['users']
    >>> db.users.relate('loans', db.loans, order_by=db.loans.loan_date, cascade='all, delete-orphan')

## Advanced Use

### Sessions, Transations and Application Integration

Note: please read and understand this section thoroughly before using SqlSoup in any web application.

SqlSoup uses a ScopedSession to provide thread-local sessions. You can get a reference to the current one like this:

    >>> session = db.session

The default session is available at the module level in SQLSoup, via:

    >>> from sqlalchemy.ext.sqlsoup import Session

The configuration of this session is `autoflush=True`, `autocommit=False`. This means when you work with the SqlSoup object, you need to call `db.commit()` in order to have changes persisted. You may also call `db.rollback()` to roll things back.

Since the SqlSoup object’s Session automatically enters into a transaction as soon as it’s used, it is essential that you call `commit()` or `rollback()` on it when the work within a thread completes. This means all the guidelines for web application integration at Lifespan of a Contextual Session must be followed.

The SqlSoup object can have any session or scoped session configured onto it. This is of key importance when integrating with existing code or frameworks such as Pylons. If your application already has a `Session` configured, pass it to your SqlSoup object:

    >>> from myapplication import Session
    >>> db = SqlSoup(session=Session)

If the `Session` is configured with `autocommit=True`, use `flush()` instead of `commit()` to persist changes - in this case, the `Session` closes out its transaction immediately and no external management is needed. `rollback()` is also not available. Configuring a new SQLSoup object in “autocommit” mode looks like:

    >>> from sqlalchemy.orm import scoped_session, sessionmaker
    >>> db = SqlSoup('sqlite://', session=scoped_session(sessionmaker(autoflush=False, expire_on_commit=False, autocommit=True)))

### Mapping arbitrary Selectables

SqlSoup can map any SQLAlchemy `Selectable` with the map method. Let’s map an `expression.select()` object that uses an aggregate function; we’ll use the SQLAlchemy `Table` that SqlSoup introspected as the basis. (Since we’re not mapping to a simple table or join, we need to tell SQLAlchemy how to find the primary key which just needs to be unique within the select, and not necessarily correspond to a real PK in the database.):

    >>> from sqlalchemy import select, func
    >>> b = db.books._table
    >>> s = select([b.c.published_year, func.count('*').label('n')], from_obj=[b], group_by=[b.c.published_year])
    >>> s = s.alias('years_with_count')
    >>> years_with_count = db.map(s, primary_key=[s.c.published_year])
    >>> years_with_count.filter_by(published_year='1989').all()
    [MappedBooks(published_year=u'1989',n=1)]

Obviously if we just wanted to get a list of counts associated with book years once, raw SQL is going to be less work. The advantage of mapping a Select is reusability, both standalone and in Joins. (And if you go to full SQLAlchemy, you can perform mappings like this directly to your object models.)

An easy way to save mapped selectables like this is to just hang them on your db object:

    >>> db.years_with_count = years_with_count

Python is flexible like that!

### Raw SQL

SqlSoup works fine with SQLAlchemy’s text construct, described in Using Text. You can also execute textual SQL directly using the execute() method, which corresponds to the execute() method on the underlying Session. Expressions here are expressed like `text()` constructs, using named parameters with colons:

    >>> rp = db.execute('select name, email from users where name like :name order by name', name='%Bhargan%')
    >>> for name, email in rp.fetchall(): print name, email
    Bhargan Basepair basepair+nospam@example.edu

Or you can get at the current transaction’s connection using connection(). This is the raw connection object which can accept any sort of SQL expression or raw SQL string passed to the database:

    >>> conn = db.connection()
    >>> conn.execute("'select name, email from users where name like ? order by name'", '%Bhargan%')

### Dynamic table names

You can load a table whose name is specified at runtime with the entity() method:

    >>> tablename = 'loans'
    >>> db.entity(tablename) == db.loans
    True

entity() also takes an optional schema argument. If none is specified, the default schema is used.

## SqlSoup API

class `sqlalchemy.ext.sqlsoup.` `SqlSoup`(engine_or_metadata, base=<type 'object'>, session=None)

  Represent an ORM-wrapped database resource.

  `__init__`(engine_or_metadata, base=<type 'object'>, session=None)

    Initialize a new `SqlSoup`.

    Parameters: | engine_or_metadata – a string database URL, `Engine` or `MetaData` object to associate with. If the argument is a `MetaData`, it should be bound to an `Engine`. base – a class which will serve as the default class for returned mapped classes. Defaults to `object`. session – a `ScopedSession` or `Session` with which to associate ORM operations for this `SqlSoup` instance. If `None`, a `ScopedSession` that’s local to this module is used.

  `bind`

    The `Engine` associated with this `SqlSoup`.

  `clear`()

    Synonym for `SqlSoup.expunge_all()`.

  `commit`()

    Commit the current transaction.

    See `Session.commit()`.

  `connection`()

    Return the current `Connection` in use by the current transaction.

  `delete`(instance)

    Mark an instance as deleted.

  `engine`

    The `Engine` associated with this `SqlSoup`.

  `entity`(attr, schema=None)

    Return the named entity from this `SqlSoup`, or create if not present.

    For more generalized mapping, see `map_to()`.

  `execute`(stmt, **params)

    Execute a SQL statement.

    The statement may be a string SQL string, an `expression.select()` construct, or an `expression.text()` construct.

  `expunge`(instance)

    Remove an instance from the `Session`.

    See `Session.expunge()`.

  `expunge_all`()

    Clear all objects from the current `Session`.

    See `Session.expunge_all()`.

  `flush`()

    Flush pending changes to the database.

    See `Session.flush()`.

  `join`(left, right, onclause=None, isouter=False, base=None, **mapper_args)

    Create an `expression.join()` and map to it.

    The class and its mapping are not cached and will be discarded once dereferenced (as of 0.6.6).

    Parameters: | left – a mapped class or table object. right – a mapped class or table object. onclause – optional “ON” clause construct.. isouter – if True, the join will be an OUTER join. base – a Python class which will be used as the base for the mapped class. If `None`, the “base” argument specified by this `SqlSoup` instance’s constructor will be used, which defaults to `object`. mapper_args – Dictionary of arguments which will be passed directly to `orm.mapper()`.

  `map`(selectable, base=None, **mapper_args)

    Map a selectable directly.

    The class and its mapping are not cached and will be discarded once dereferenced (as of 0.6.6).

    Parameters: | selectable – an `expression.select()` construct. base – a Python class which will be used as the base for the mapped class. If `None`, the “base” argument specified by this `SqlSoup` instance’s constructor will be used, which defaults to `object`. mapper_args – Dictionary of arguments which will be passed directly to `orm.mapper()`.

  `map_to`(attrname, tablename=None, selectable=None, schema=None, base=None, mapper_args=immutabledict({}))

    Configure a mapping to the given attrname.

    This is the “master” method that can be used to create any configuration.

    (new in 0.6.6)

    Parameters: | attrname – String attribute name which will be established as an attribute on this :class:.`.SqlSoup` instance. base – a Python class which will be used as the base for the mapped class. If `None`, the “base” argument specified by this `SqlSoup` instance’s constructor will be used, which defaults to `object`. mapper_args – Dictionary of arguments which will be passed directly to `orm.mapper()`. tablename – String name of a `Table` to be reflected. If a `Table` is already available, use the `selectable` argument. This argument is mutually exclusive versus the `selectable` argument. selectable – a `Table`, `Join`, or `Select` object which will be mapped. This argument is mutually exclusive versus the `tablename` argument. schema – String schema name to use if the `tablename` argument is present.

  `rollback`()

    Rollback the current transction.

    See `Session.rollback()`.

  `with_labels`(selectable, base=None, **mapper_args)

    Map a selectable directly, wrapping the selectable in a subquery with labels.

    The class and its mapping are not cached and will be discarded once dereferenced (as of 0.6.6).

    Parameters: | selectable – an `expression.select()` construct. base – a Python class which will be used as the base for the mapped class. If `None`, the “base” argument specified by this `SqlSoup` instance’s constructor will be used, which defaults to `object`. mapper_args – Dictionary of arguments which will be passed directly to `orm.mapper()`.