half-orm 0.5.4

A simple PostgreSQL-Python relation-object mapper.

A simple PostgreSQL-Python relation-object mapper.

half_orm maps an existing PostgreSQL database to Python objects with inheritance as defined in PostgreSQL.

Why half?

The SQL language is divided in two different parts:

• the data definition language part (DDL) to manipulate the structure of a database,
• the data manipulation language part (DML) used for selecting, inserting, deleting and updating data in a database.

half_orm only deals with the DML part. Basically the INSERT, SELECT, UPDATE and DELETE commands. This makes half_orm easy to learn and use. In a way, half_orm is more a ROM (relation-object mapper) than an ORM.

Learn half_orm in half an hour

You have a PostgreSQL database ready at hand (you can try half_orm with pagila)

Install half_orm

run pip install half_orm in a virtual environment.

Create a directory to store your connection files and set the shell variable HALFORM_CONF_DIR:

% mkdir ~/.half_orm
% export HALFORM_CONF_DIR=~/.half_orm

Create a connection file in the HALFORM_CONF_DIR containing the following information (with your values):

[database]
name = db_name
user = username
password = password
host = localhost
port = 5432

Your ready to go!

Connect to the database

>>> from half_orm.model import Model
>>> my_db = Model('my_database') # OK wrong naming... this is a Pg database ;)

The my_database is the name of the connexion file. It will be fetched in the directory referenced by the shell variable HALFORM_CONF_DIR if defined, in /etc/half_orm otherwise.

Get a rapid description of the database structure

Once connected to the database, you can easily have an overview of its structure:

print(my_db)

It displays as many lines as there are relations, views or materialized views in your database. Each row has the form:

<relation type> <"schema name"."relation name">

Where relation type is one of r, p, v, m, f:

• r for a relation,
• p for a partitioned table,
• v for a view,
• m for a materialized view,
• f for foreign data.

for instance (using the halftest database):

r "actor"."person"
r "blog"."comment"
r "blog"."event"
r "blog"."post"
v "blog.view"."post_comment"

Check if a relation exists in the database

>>> my_db.has_relation('blog.view.post_comment')
True

Get the class of a relation (the Model.get_relation_class method)

To work with a table of your database, you must instanciate the corresponding class:

Person = halftest.get_relation_class('actor.person')
PostComment = halftest.get_relation_class('blog.view.post_comment')

The argument passed to get_relation_class is as string of the form: <schema_name>.<relation_name>. Note that while dots are allowed in the schemas, this is not the case for relations.

To get a full description of the corresponding relation, print an instance of the class:

>>> print(Person())
__RCLS: <class 'half_orm.relation.Table_HalftestActorPerson'>
This class allows you to manipulate the data in the PG relation:
TABLE: "halftest"."actor"."person"
DESCRIPTION:
The table actor.person contains the persons of the blogging system.
The id attribute is a serial. Just pass first_name, last_name and birth_date
to insert a new person.
FIELDS:
- id:         (int4) UNIQUE NOT NULL
- first_name: (text) PK
- last_name:  (text) PK
- birth_date: (date) PK
FOREIGN KEYS:
- _reverse_fkey_halftest_blog_comment_author_id: ("id")
 ↳ "halftest"."blog"."comment"(author_id)
- _reverse_fkey_halftest_blog_event_author_first_name_author_last_name_author_birth_date: ("birth_date", "first_name", "last_name")
 ↳ "halftest"."blog"."event"(author_first_name, author_last_name, author_birth_date)
- _reverse_fkey_halftest_blog_post_author_first_name_author_last_name_author_birth_date: ("birth_date", "first_name", "last_name")
 ↳ "halftest"."blog"."post"(author_first_name, author_last_name, author_birth_date)```

It provides you with information extracted from the database metadata:

• description: the comment on the relationship if there is one,
• fields: the list of columns, their types and contraints
• foreign keys: the list of FKs if any. A _reverse_* FK is a FK made on the current relation.

Constraining a relation

when you instantiate an object with no arguments, its intention corresponds to all the data present in the corresponding relation. Person() represents the set of people contained in the actor.person table (ie. there is no constraint on the set). You can get the number of elements in a relation whith the len function as in len(Person()).

To constrain a set, you must specify one or more values for the fields/columns in the set with a tuple of the form: (comp, value). comp (= if ommited) is either a comparison operator or a pattern matching operator (like or POSIX regular expression).

You can constrain a relation object at instanciation:

Person(last_name='Lagaffe', first_name='Gaston', birth_date='1957-02-28')
Person(last_name=('ilike', '_a%'))
Person(birth_date='1957-02-28')

You can also constrain an instanciated object:

gaston = Person()
gaston.last_name = ('ilike', 'l%')
gaston.first_name = 'Gaston'

half_orm prevents you from making typos:

gaston.lost_name = 'Lagaffe'
# raises a half_orm.relation_errors.IsFrozenError Exception

The NULL value

half_orm provides the NULL value:

from half_orm.null import NULL

nobody = Person()
nobody.last_name = NULL
assert len(nobody) == 0 # last_name is part of the PK

Set operators

You can use the set operators to set more complex constraints on your relations:

• &, |, ^ and - for and, or, xor and not. Take a look at the algebra test file.
• you can also use the ==, != and in operators to compare two sets.

my_selection = Person(last_name=('ilike', '_a%'))
my_selection |= Person(first_name=('ilike', 'A%'))

my_selection represents the set of persons whose second letter of the name is an a or whose first letter of the first name is an a.

DML. The insert, select, update,delete methods.

These methods trigger their corresponding SQL querie on the database. For debugging purposes, you can activate the print the SQL query built by half_orm when the DML method is invoked using the _mogrify() method.

persons._mogrify()
persons.select()

Insert

To insert a tuple in the relation, use the insert method as shown bellow:

Person(last_name='Lagaffe', first_name='Gaston', birth_date='1957-02-28').insert()

You can put a transaction on any function using the Relation.Transaction decorator.

persons = Person()

@persons.Transaction
def insert_many(persons, data):
    for person in data:
        persons(**person).insert()

insert_many(persons, data=[
    {'last_name':'Lagaffe', 'first_name':'Gaston', 'birth_date':'1957-02-28'},
    {'last_name':'Fricotin', 'first_name':'Bibi', 'birth_date':'1924-10-05'},
    {'last_name':'Maltese', 'first_name':'Corto', 'birth_date':'1975-01-07'},
    {'last_name':'Talon', 'first_name':'Achile', 'birth_date':'1963-11-07'},
    {'last_name':'Jourdan', 'first_name':'Gil', 'birth_date':'1956-09-20'}
])

Notice:

• half_orm works in autocommit mode by default.
• if "Lagaffe" was already inserted, none of the data would be inserted by insert_many.

Select

The select method is a generator. It returns all the data of the relation that match the constraint defined on the Relation object. The data is returned in a list of dictionaries.

>>> persons = Person()
>>> for pers in persons.select():
...     print(pers)
...
{'first_name': 'Gaston', 'birth_date': datetime.date(1957, 2, 28), 'id': 159361, 'last_name': 'Lagaffe'}
{'first_name': 'Bibi', 'birth_date': datetime.date(1924, 10, 5), 'id': 159362, 'last_name': 'Fricotin'}
{'first_name': 'Corto', 'birth_date': datetime.date(1975, 1, 7), 'id': 159363, 'last_name': 'Maltese'}
{'first_name': 'Achile', 'birth_date': datetime.date(1963, 11, 7), 'id': 159364, 'last_name': 'Talon'}
{'first_name': 'Gil', 'birth_date': datetime.date(1956, 9, 20), 'id': 159365, 'last_name': 'Jourdan'}
>>>

You can set a limit or an offset:

>>> persons.offset(2).limit(3)

>>> _a_persons = Person(last_name=('like',  '_a%'))

>>> [elt['last_name'] for elt in _a_persons.select()]
['Lagaffe', 'Maltese', 'Talon']

You can also get a subset of the attributes:

>>> for dct in _a_persons.select('last_name'):
...     print(dct)
{'last_name': 'Lagaffe'}
{'last_name': 'Maltese'}
{'last_name': 'Talon'}

Select one: the get method

the get method returns an object whose fields are constrained with the values of the corresponding row in the database. It raises an ExpectedOneError Exception if 0 or more than 1 rows match the intention.

gaston = Person(last_name='Lagaffe').get()

is equivalent to

people = Person(last_name='Lagaffe')
if people.is_empty() or len(people) > 1:
    raise ExcpetedOneError
gaston = Person(**next(people.select()))

Update

To update a subset, you first define the subset an then invoque the udpate method with the new values passed as argument.

In the following example, we capitalize the last name of all people whose second letter is an a.

@persons.Transaction
def upper_a(persons):
    for d_pers in persons.select():
        pers = Person(id=d_pers['id'])
        pers.update(last_name=d_pers['last_name'].upper())

upper_a(_a_persons)

WARNING! The following code is perfectly legitimate, but it won't work. _a_persons.select() returns a list of dictionaries and the dict.update method would only update the corresponding dictonary. It's a common pitfall.

@persons.Transaction
def upper_a(persons):
    # Won't work!
    for pers in persons.select():
        pers.update(last_name=pers['last_name'].upper())

upper_a(_a_persons)

Again, we insure the atomicity of the transaction using the Relation.Transaction decorator.

>>> [elt['last_name'] for elt in Person(last_name=('like', '_A%')).select()]
['LAGAFFE', 'MALTESE', 'TALON']

If you want to update all the data in a relation, you must set the argument update_all to True.

Delete

We finally remove every inserted tuples. Note that we use the delete_all argument with a True value. The delete would have been rejected otherwise:

>>> persons().delete(delete_all=True)
>>> list(persons().select())
[]

Well, there is not much left after this in the actor.person table.

Working with foreign keys (the FKey class)

Working with foreign keys is as easy as working with Relational objects. A Relational object has an attribute _fkeys that contains the foreign keys in a dictionary. Foreign keys are Fkey objects. The Fkey class has one method:

• the set method allows you to constrain a foreign key with a Relation object,
• a foreign key is a transitional object, so when you "call" an FKey object, you get the relation it points to. The original constraint is propagated through the foreign key.

Let's see an example with the blog.comment relation:

>>> Comment = halftest.get_relation_class('blog.comment')
>>> Comment()
__RCLS: <class 'half_orm.relation.Table_HalftestBlogComment'>
This class allows you to manipulate the data in the PG relation:
TABLE: "halftest"."blog"."comment"
DESCRIPTION:
The table blog.comment contains all the comments
made by a person on a post.
FIELDS:
- id:        (int4) PK
- content:   (text)
- post_id:   (int4)
- author_id: (int4)
- a = 1:     (text)
FOREIGN KEYS:
- post: ("post_id")
 ↳ "halftest"."blog"."post"(id)
- author: ("author_id")
 ↳ "halftest"."actor"."person"(id)

It has two foreign keys named post and author.

We want the comments made by Gaston:

gaston = Person(last_name="Lagaffe")
gaston_comments = Comment()
gaston_comments._fkeys['author'].set(gaston)

To know on which posts gaston made at least one comment, we just "call" the foreign key post on gaston_comments:

>>> the_posts_commented_by_gaston = gaston_comments._fkeys['post']()
>>> isinstance(the_posts_commented_by_gaston, halftest.get_relation_class('blog.post'))
True

Knowing that a Post object has the following structure:

TABLE: "halftest"."blog"."post"
DESCRIPTION:
The table blog.post contains all the post
made by a person in the blogging system.
FIELDS:
- id:                (int4) PK
- title:             (text)
- content:           (text)
- author_first_name: (text)
- author_last_name:  (text)
- author_birth_date: (date)
FOREIGN KEYS:
- author: (author_first_name, author_last_name, author_birth_date)
 ↳ "halftest"."actor"."person"(first_name, last_name, birth_date)

We can now retreive the authors of the_posts_commented_by_gaston:

>>> the_authors_of_posts_commented_by_gaston = the_posts_commented_by_gaston._fkeys['author']()
>>> list(the_authors_of_posts_commented_by_gaston.select())
[...]

Again, the_authors_of_posts_commented_by_gaston is an object of the class halftest.get_relation_class('actor.person'). It's that easy!

the reverse Fkeys

With half_orm you can also go upstream. If we look at the foreign keys of the Person class, they are all prefixed by _reverse_. This means that the actor.person table is referenced by other tables:

FOREIGN KEYS:
- _reverse_fkey_halftest_blog_comment_author_id: ("id")
 ↳ "halftest"."blog"."comment"(author_id)
- _reverse_fkey_halftest_blog_event_author_first_name_author_last_name_author_birth_date: ("birth_date", "first_name", "last_name")
 ↳ "halftest"."blog"."event"(author_first_name, author_last_name, author_birth_date)
- _reverse_fkey_halftest_blog_post_author_first_name_author_last_name_author_birth_date: ("birth_date", "first_name", "last_name")
 ↳ "halftest"."blog"."post"(author_first_name, author_last_name, author_birth_date)

If we wanted to recover the posts, events and comments made by Gaston, we would just have to write:

gaston = Person(last_name='Lagaffe', first_name='Gaston')
# assuming there is only one Gaston Lagaffe
g_comments = gaston._fkeys['_reverse_fkey_halftest_blog_comment_author_id']()
g_events = gaston._fkeys['_reverse_fkey_halftest_blog_event_author_first_name_author_last_name_author_birth_date']()
g_posts = gaston._fkeys['_reverse_fkey_halftest_blog_post_author_first_name_author_last_name_author_birth_date']()

Last: SQL queries

If you realy need to invoke a SQL query not managed by half_orm, use the Model.execute_query method:

from half_orm.model import Model
halftest = Model('halftest')
halftest.execute_query('select 1')

By the way, this is the code used in the Model.ping method that makes sure the connection is established and attempts a reconnection if it is not.

That's it! You've learn pretty much everything there is to know with half_orm.

Next: hop, the half_orm packager

The hop command, provided by the package half_orm_packager, allows you to create a Python package corresponding to the model of your database, to patch the model and the corresponding Python code, to test your database model and your business code. For more information, see https://github.com/collorg/halfORM_packager.

Want to contribute?

Fork me on Github: https://github.com/collorg/halfORM