Author Archives: Alexey Malashkevich

Pony ORM Release 0.5.4

New functions and methods:

  • pony.orm.serialization module with the to_dict() and to_json() functions was added. Before this release you could use the to_dict() method of an entity instance in order to get a key-value dictionary structure for a specific entity instance. Sometimes you might need to serialize not only the instance itself, but also the instance’s related objects. In this case you can use the to_dict() function from the pony.orm.serialization module.

    • to_dict() – receives an entity instance or a list of instances and returns a dictionary structure which keeps the passed object(s) and immediate related objects
    • to_json() – uses `to_dict()` and returns JSON representation of the to_dict() result
  • Query.prefetch() – allows to specify which related objects or attributes should be loaded from the database along with the query result . Example:

          select(s for s in Student)\
              .prefetch(Group, Department,
  • obj.flush() – allows flush a specific entity to the database
  • obj.get_pk() – return the primary key value for an entity instance
  • py_check parameter for attributes added. This parameter allows you to specify a function which will be used for checking the value before it is assigned to the attribute. The function should return True/False or can raise ValueError exception if the check failed. Example:

        class Student(db.Entity):
            name = Required(unicode)
            gpa = Required(float, py_check=lambda v: v >= 0 and v <= 5)

    New types:

    • time and timedelta – now you can use these types for attribute declaration. Also you can use timedelta and a combination of datetime + timedelta types inside queries.

    New hooks:

    • after_insert, after_update, after_delete - these hooks are called when an object was inserted, updated or deleted in the database respectively (link)
    • Added support for pymysql – pure Python MySQL client. Currently it is used as a fallback for MySQLdb interface

    Other changes and bug fixes

    • obj.order_by() method is deprecated, use instead
    • obj.describe() now displays composite primary keys
    • Fixes #50: PonyORM does not escape _ and % in LIKE queries
    • Fixes #51: Handling of one-to-one relations in declarative queries
    • Fixes #52: An attribute without a column should not have rbits & wbits
    • Fixes #53: Column generated at the wrong side of "one-to-one" relationship
    • Fixes #55: obj.to_dict() should do flush at first if the session cache is modified
    • Fixes #57: Error in to_dict() when to-one attribute value is None
    • Fixes #70: EntitySet allows to add and remove None
    • Check that the entity name starts with a capital letter and throw exception if it is not then raise the ERDiagramError: Entity class name should start with a capital letter exception

Pony ORM Release 0.5.3

This release fixes the problem that was found after the previous release was uploaded to PyPI.

You can install the latest Pony ORM version using pip:

    pip install pony

Or, if you already have the previous version of Pony ORM installed, upgrade it:

    pip install --upgrade pony

Pony ORM Release 0.5.2

This release is a step forward to Python 3 support. While the external API wasn’t changed, the internals were significantly refactored to provide forward compatibility with Python 3.

Changes since 0.5.1

  • New Entity instance method
    to_dict(only=None, exclude=None, with_collections=False, with_lazy=False, related_objects=False)

    Returns a dictionary with attribute names and its values. This method can be used when you need to serialize an object to JSON or other format.

    By default this method doesn’t include collections (relationships to-many) and lazy attributes. If an attribute’s values is an entity instance then only the primary key of this object will be added to the dictionary.

    only – use this parameter if you want to get only the specified attributes. This argument can be used as a first positional argument. You can specify a list of attribute names obj.to_dict(['id', 'name']), a string separated by spaces: obj.to_dict('id name'), or a string separated by spaces with commas: obj.to_dict('id, name').

    exclude – this parameter allows to exclude specified attributes. Attribute names can be specified the same way as for the only parameter.

    related_objects – by default, all related objects represented as a primary key. If related_objects=True, then objects which have relationships with the current object will be added to the resulting dict as objects, not their primary keys. It can be useful if you want to walk the related objects and call the to_dict() method recursively.

    with_collectionsTrue, then the relationships to-many will be represented as lists. If related_objects=False (which is by default), then those lists will consist of primary keys of related instances. If related_objects=True then to-many collections will be represented as lists of objects.

    with_lazy – if True, then lazy attributes (such as BLOBs or attributes which are declared with lazy=True) will be included to the resulting dict.

    For illustrating the usage of this method we will use the eStore example which comes with Pony distribution. Let’s get a customer object with the id=1 and convert it to a dictionary:

           >>> from pony.orm.examples.estore import *
           >>> c1 = Customer[1]
           >>> c1.to_dict()
           {'address': u'address 1',
           'country': u'USA',
           'email': u'',
           'id': 1,
           'name': u'John Smith',
           'password': u'***'}

    If we don’t want to serialize the password attribute, we can exclude it this way:

           >>> c1.to_dict(exclude='password')
           {'address': u'address 1',
           'country': u'USA',
           'email': u'',
           'id': 1,
           'name': u'John Smith'}

    If you want to exclude more than one attribute, you can specify them as a list: exclude=['id', 'password'] or as a string: exclude='id, password' which is the same as exclude='id password'.

    Also you can specify only the attributes, which you want to serialize using the parameter only:

           >>> c1.to_dict(only=['id', 'name'])
           {'id': 1, 'name': u'John Smith'}
           >>> c1.to_dict('name email') # 'only' parameter as a positional argument
           {'email': u'', 'name': u'John Smith'}

    By default the collections are not included to the resulting dict. If you want to include them, you can specify with_collections=True. Also you can specify the collection attribute in the only parameter:

           >>> c1.to_dict(with_collections=True)
          {'address': u'address 1',
           'cart_items': [1, 2],
           'country': u'USA',
           'email': u'',
           'id': 1,
           'name': u'John Smith',
           'orders': [1, 2],
           'password': u'***'}

    By default all related objects (cart_items, orders) are represented as a list with their primary keys. If you want to see the related objects instances, you can specify related_objects=True:

           >>> c1.to_dict(with_collections=True, related_objects=True)
           {'address': u'address 1',
           'cart_items': [CartItem[1], CartItem[2]],
           'country': u'USA',
           'email': u'',
           'id': 1,
           'name': u'John Smith',
           'orders': [Order[1], Order[2]],
           'password': u'***'}


  • Now select() function and filter() method of the Query object can accept lambdas with closures
  • Some minor bugs were fixed

You can install the latest Pony ORM version using pip:

    pip install pony

Or, if you already have the previous version of Pony ORM installed, upgrade it:

    pip install --upgrade pony

Pony ORM at EuroPython 2014

Recently we had the “How Pony ORM translates Python generators to SQL queries” session at EuroPython in Berlin.
In this talk we shared interesting implementation details of our mapper. Also there is a little bit of comparison with other ORMs. Thanks to everyone who was there and for your feedback!

Here is the video from the conference:

And here are the slides:

Pony ORM Release 0.5.1

Before this release, if a text attribute was defined without the max length specified (e.g. name = Required(unicode)), Pony set the maximum length equal to 200 and used SQL type VARCHAR(200).

Actually, PostgreSQL and SQLite do not require specifying the maximum length for strings. Starting with this release such text attributes are declared as TEXT in SQLite and PostgreSQL. In these DBMSes, the TEXT datatype has the same performance as VARCHAR(N) and doesn’t have arbitrary length restrictions.

For other DBMSes default VARCHAR limit was increased up to 255 in MySQL and to 1000 in Oracle.


  • Correct parsing of datetime values with the ‘T’ separator between date and time
  • Entity.delete() bug fixed
  • Lazy attribute loading bug fixed

Pony ORM Release 0.5

We are proud to announce the Pony ORM Release 0.5! This release brings lots of improvements and new features which are listed below.

Changes since 0.4.9

  • New transaction model (link)
  • New method Query.filter() allows step-by-step query construction (link)
  • New method Database.bind() simplifies testing and allows using different settings for development and production (link)
  • New method simplifies pagination (link)
  • New method MyEntity.select_random(N) is effective for large tables (link)
  • New method Query.random(N) for selecting random instances (link)
  • Support of new concat() function inside declarative queries
    New before_insert(), before_update(), before_delete() entity instance hooks which can be overridden
  • Ability to specify sequence_name=’seq_name’ for PrimaryKey attributes in Oracle database
  • Ability to create new entity instances specifying the value of the primary key instead of the object
  • Ability to read entity object attributes outside of the db_session
  • Ability to use lambdas as a reference to an entity in relationship attribute declarations (link)
  • The names of tables, indexes and constraints in the database creation script now are sorted in the alphabetical order
    In MySQL and PostgreSQL Pony converts the table names to the lower case. In Oracle – to the upper case. In SQLite leaves as is.
  • The option options.MAX_FETCH_COUNT is set to None by default now
  • The support of PyGreSQL is discontinued, using psycopg2 instead
  • Added pony.__version__ attribute
  • Multiple bugs were fixed
  • Stability and performance improvements

Pony ORM Release 0.5rc1

  • Before this release Pony used the exact letter case of an entity for the MySQL database table names (unless you specify the table name explicitly using the _table_ option).
    This approach turned out to be not reliable on all platforms: “MySQL table names are case-sensitive depending on the filesystem of the server. Since Windows and Mac are a case-insensitive OS, and Linux is case-sensitive; the MySQL database treats the uppercase table name differently than lowercase table name on Linux machines.”

    The official MySQL documentation reads:
    To avoid problems caused by such differences, it is best to adopt a consistent convention, such as always creating and referring to databases and tables using lowercase names. This convention is recommended for maximum portability and ease of use. Following this recommendation, starting with this release, Pony converts all the table names during its creation in accordance with the style of the respective database system. In MySQL and PostgreSQL Pony converts the table names to the lower case. In Oracle – to the upper case. In SQLite there is no need to convert the letter case – SQLite works with the table name letter case equally on all platforms.In order to make the new release compatible with your tables created with the previous Pony releases you have a couple of options:

    • Specify the table names for each entity explicitly:
          class MyEntity(db.Entity):
              _table_ = "MyEnity"
    • Convert the names of your tables to the lower case using SQL commands manually:
          RENAME TABLE `MyEntity` TO `myentity`
  • Starting with this release the names of tables, indexes and constraints in the database creation script are sorted in the alphabetical order.
  • Now you can use lambdas instead of parenthesis for the entities which are declared later in your code:
        class User(db.Entity):
            name = Required(unicode)
            photos = Set(lambda: Photo)
        class Photo(db.Entity):
            content = Required(buffer)
            user = Required(User)

    This can be useful if you want your IDE to check the names of declared entities and highlight typos.

  • Instances now have the before_insert, before_update, before_delete hooks:
        class MyEntity(db.Entity):
            def before_insert(self):
                print '%s is about to be inserted!' % self
            def before_update(self):
                print '%s is about to be updated!' % self
            def before_delete(self):
                print '%s is about to be deleted!' % self
  • Now options.MAX_FETCH_COUNT is set to None by default. Before this release Pony would throw the TooManyRowsFound exception if the number of instances returned by a query exceeds the MAX_FETCH_COUNT. Since this parameter can vary greatly, Pony doesn’t set it by default anymore.
  • We have added the concat() function and now you can do the following:
    select(concat(, ':', s.dob.year, ':', s.scholarship) 
           for s in Student)
  • With the new Pony release you have the read-only access to the entity instances outside of the db_session. Before this release any attempt to use an instance outside of the db_session would result in a TransactionRolledBack exception. Now you can get the attribute values, but if the attribute that you’re trying to access was not loaded, then you’ll get the DatabaseSessionIsOver exception. The same exception will be raised in case you want to change attribute values.
    We hope that this option will not be misused. Most of the time you want to use entity instances within the db_session. If you find yourself using the instances outside of the db_session often, you probably doing something strange.
  • Now when you create new instances you can use the value of the primary key instead of the object. This is how you created an instance of a Student before:
        student = Student(name='Student name', group=Group[123])

    and now you can pass a primary key of the Group object instead:

        student = Student(name='Student name', group=123)

    The same approach can be used for the get() method.

  • Now you can specify the sequence name for PrimaryKey attributes for Oracle databases using the keyword argument sequence_name='seq_name'
  • The bug #41 was fixed. Now you can use the same variable name in subsequent filters and it can have different values.
  • Numerous other bugs were fixed

Pony ORM Release 0.5-beta

New transaction model

Before this release Pony supported two separate transaction modes: standard and optimistic. In this release we’ve combined both of them into one. Now user can choose which locking mechanism should be use for any specific object. By default Pony uses the optimistic locking. This means, that the database doesn’t lock any objects in the database. In order to be sure that those objects weren’t updated by a concurrent transaction, Pony checks the corresponding database rows during update.

Sometimes a user might want to go with the pessimistic locking. With this approach the database locks the corresponding rows for the exclusive access in order to prevent concurrent modification. It can be done with the help of for update() methods. In this case there is no need for Pony to do the optimistic check. You can use any of the following methods:

    prod_list = select(p for p in Product if p.price > 100).for_update()
    prod_list = p: p.price > 100).for_update()
    p = Product.get_for_update(id=123)

Pony ORM transaction processing is described in more detail in the newly created documentation chapter Transactions.


In this release we have added the ability to add comments to our online documentation, please leave your feedback, questions, comments. We need you in order to make Pony ORM documentation better.

Filtering the query results

With this release we introduce a new method of the Query object: filter(). It can be convenient for applying conditions for an existing query. You can find more information in the documentation

Database late binding

Pony ORM users were asking us to provide a way to bind the database at a later stage. This is convenient for testing. The goal of Pony ORM team is to provide a convenient tool for working with databases and in this release we’ve added such a possibility.

Before this release the process of mapping entities to the database was as following:

    db = Database('sqlite', 'database.sqlite')

    class Entity1(db.Entity):

    # other entities declaration


Now besides the approach described above, you can use the following way:

    db = Database()

    class Entity1(db.Entity):

    # other entities declaration

    db.bind('sqlite', 'database.sqlite')

Other features and bugfixes

  • Added method for pagination
  • Added count() method for collections. E.g. Customer[123].orders.count()
  • Added ORDER BY RANDOM(): MyEntity.select_random(N),, select(...).random(N) – N is the number of elements to be selected
  • Bugfix in exists() subquery
  • Bugfix when the same item was added and removed within the same transaction
  • Aggregations bugfix
  • The support of PyGreSQL is discontinued. Using psycopg2 instead
  • Added pony.__version__ attribute
  • Stability and performance improvements

Now you can create private ER diagrams with PonyORM Diagram Editor

We are excited to announce the launch of our new PonyORM Diagram Editor website. It is located at the same address:

The main new features include:

  • The ability to create private ER diagrams. Such diagrams can be viewed only by the owner of the diagram and users which were invited by the diagram owner.
  • Revision history. This feature allows the user to track changes and restore a diagram to a previous version.
  • Now you can share your diagram with other people sending invitations from the website.

You can see the available plans here: