half-orm 0.13.2

A simple PostgreSQL to Python mapper.

half_orm: A simple PostgreSQL to Python mapper

Nowadays, most applications require interacting with a relational database. While full-fledged ORMs like SQLAlchemy are very powerful, their complexity, steep learning curve, and some of their limitations can be a hindrance. This is the context in which half_orm was born, a minimalist ORM specifically designed for PostgreSQL. The main motivation is to allow modeling the database directly in SQL, taking full advantage of the capabilities offered by the PostgreSQL engine (triggers, views, functions, stored procedures, inheritance handling...), while avoiding the "impedance mismatch" issues, loss of control over generated SQL, and rigidity encountered with full-fledged ORMs that model the schema at the object level. Half_orm intentionally excludes the DDL aspects (schema creation) of the SQL language. The goal is to provide a lightweight abstraction layer over standard SQL queries while maintaining transparent access to the underlying database engine. With half_orm, writing INSERT, SELECT, UPDATE, and DELETE queries becomes as simple as with SQL, but in the comfort of Python. Its operation aims to be intuitive thanks to a lean API that emphasizes productivity and code readability.

Here is what coding with half_orm looks like :

from half_orm.model import Model
from half_orm.relation import singleton

halftest = Model('halftest') # We connect to the PostgreSQL database
# print(halftest) to get the list of relations in the database

class Post(halftest.get_relation_class('blog.post')):
    """blog.post is a table of the halftest database (<schema>.<relation>)
    To get a full description of the relation, use print(Post())
    """
    #  The Fkeys template is provided by print(Post()). Just fill in the keys names.
    Fkeys = {
        'comments_rfk': '_reverse_fkey_halftest_blog_comment_post_id', # a post is referenced by comments
        'author_fk': 'author' # the post references a person
    }

class Person(halftest.get_relation_class('actor.person')):
    Fkeys = {
        'posts_rfk': '_reverse_fkey_halftest_blog_post_author_first_name_author_last_name_author_birth_date',
        'comments_rfk': '_reverse_fkey_halftest_blog_comment_author_id'
    }
    @singleton # This ensures that the author of the post is well defined.
    def add_post(self, title: str=None, content: str=None) -> dict:
        return self.posts_rfk(title=title, content=content).ho_insert()
    @singleton
    def add_comment(self, post: Post=None, content: str=None) -> dict:
        return self.comments_rfk(content=content, post_id=post.id.value).ho_insert()

def main():
    gaston = Person(last_name='Lagaffe', first_name='Gaston', birth_date='1957-02-28')
    gaston.ho_delete()
    if gaston.ho_is_empty(): # gaston defines a subset of the actor.person table.
        gaston.ho_insert()
    post = Post(**gaston.add_post(title='Easy', content='halfORM is fun!'))
    gaston.add_comment(content='This is a comment on the newly created post.', post=post)
    print(list(post.comments_rfk())) # The relational objects are iterators
    post.ho_update(title='Super easy')
    gaston.ho_delete()

If you want to build or patch a model, see the half_orm packager.

Tutorial: Learn half_orm in half an hour

Install half_orm

run pip install half_orm in a virtual environment.

Set your HALFORM_CONF_DIR

Create a directory to store your connection files and set the shell variable HALFORM_CONF_DIR (by default, half_orm looks in the /etc/half_orm directory):

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

Set your HALFORM_CONF_DIR for windows users:

• select settings in the menu
• search for "variable"
• select "Edit environment variables for your account"

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

You are ready to go!

Connect to the database

>>> from half_orm.model import Model
>>> my_db = Model('my_database')

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

Get a rapid description of the database structure

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

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"

Note: We only allow dots in schema names.

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:

class Person(halftest.get_relation_class('actor.person')):
    pass
class PostComment(halftest.get_relation_class('blog.view.post_comment')):
    pass

The argument passed to get_relation_class is as string of the form: <schema_name>.<relation_name>.

Note: Again, dots are only allowed in schema names.

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

>>> print(Person())

DATABASE: halftest
SCHEMA: actor
TABLE: 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) NOT NULL
- first_name: (text) NOT NULL
- last_name:  (text) NOT NULL
- birth_date: (date) NOT NULL

PRIMARY KEY (first_name, last_name, birth_date)
UNIQUE CONSTRAINT (id)
UNIQUE CONSTRAINT (first_name)
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: ("first_name", "last_name", "birth_date")
 ↳ "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: ("first_name", "last_name", "birth_date")
 ↳ "halftest":"blog"."post"(author_first_name, author_last_name, author_birth_date)

To use the foreign keys as direct attributes of the class, copy/paste the Fkeys below into
your code as a class attribute and replace the empty string key(s) with the alias(es) you
want to use. The aliases must be unique and different from any of the column names. Empty
string keys are ignored.

Fkeys = {
    '': '_reverse_fkey_halftest_blog_comment_author_id',
    '': '_reverse_fkey_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',
}

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 intent corresponds to all the data present in the corresponding relation. Person() represents the set of persons contained in the actor.person table (i.e., there is no constraint on the set).

To define a subset, you need to specify constraints on the values of the fields/columns:

• with a single value for an exact match,
• with a tuple of the form (comp, value) otherwise. The comp value is either a SQL 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 using the Field.set method:

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

half_orm prevents you from making typos:

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

Cardinality of a set

[BREAKING CHANGE] From version 0.12 onward, the __len__ method has been deprecated. It has been replaced by the ho_count method.

The code len(Person()) must be replaced by Person().ho_count().

The problem was that the Python list builtin function triggers the __len__ method if it exists. So the code list(Person()) was triggering two requests on the database : frist a SQL select count (which can be slow in PostgreSQL) and then the SQL select.

You can get the number of elements in a relation with the ho_count method, as in Person().ho_count().

The NULL value

half_orm provides the NULL value:

from half_orm.null import NULL

nobody = Person()
nobody.last_name.set(NULL)
assert nobody.ho_count() == 0 # last_name is part of the PK
[...]

The None value, unsets a constraint on a field:

[...]
nobody.last_name.set(None)
assert nobody.ho_count() == Person().ho_count()

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%')) | Person(first_name=('like', 'A%'))

my_selection represents the set of people whose second letter of the name is in ['a', 'A'] or whose first letter of the first name is an A.

DML. The ho_insert, ho_select, ho_update, ho_delete methods.

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

people.ho_mogrify()
people.ho_select()

ho_insert

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

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

By default, ho_insert returns the inserted row as a dict:

lagaffe = Person(last_name='Lagaffe', first_name='Gaston', birth_date='1957-02-28')
lagaffe_id = lagaffe.ho_insert()['id']

As of version 0.13, the Relation.ho_transaction decorator is deprecated and replaced by the Transaction(<model>) context manager, as shown in the following example:

from half_orm.transaction import Transaction
# [...]

class Person(halftest.get_relation_class('actor.person')):
    # [...]

    def insert_many(self, *data):
        """Insert serveral people in a single transaction."""

        def insert(self, *data):
            for d_pers in data:
                self(**d_pers).ho_insert()

        with Transaction(halftest):
            insert(self, *data)

people = Person()
people.insert_many(*[
    {'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'}
])

Note: half_orm works in autocommit mode by default. Without a transaction, any missing data would be inserted.

Returned values

By default ho_insert returns all the inserted values as a dictionary. You can specify the columns you want to get by passing their names as argurments to ho_insert.

ho_select

The ho_select method is a generator. It returns all the data of the relation that matches the constraint defined on the Relation object. The data is returned in a list of dicts.

>>> people = Person()
>>> print(list(people.ho_select()))
[{'id': 6753, 'first_name': 'Gaston', 'last_name': 'Lagaffe', 'birth_date': datetime.date(1957, 2, 28)}, {'id': 6754, 'first_name': 'Bibi', 'last_name': 'Fricotin', 'birth_date': datetime.date(1924, 10, 5)}, {'id': 6755, 'first_name': 'Corto', 'last_name': 'Maltese', 'birth_date': datetime.date(1975, 1, 7)}, {'id': 6756, 'first_name': 'Achile', 'last_name': 'Talon', 'birth_date': datetime.date(1963, 11, 7)}, {'id': 6757, 'first_name': 'Gil', 'last_name': 'Jourdan', 'birth_date': datetime.date(1956, 9, 20)}]
>>> print(list(people.ho_select('id', 'last_name')))
[{'id': 6753, 'last_name': 'Lagaffe'}, {'id': 6754, 'last_name': 'Fricotin'}, {'id': 6755, 'last_name': 'Maltese'}, {'id': 6756, 'last_name': 'Talon'}, {'id': 6757, 'last_name': 'Jourdan'}]
>>>

You can set a limit or an offset:

>>> people.ho_offset(1).ho_limit(2)
>>> print(list(people)) # Relation objects are iterators. so ho_select is optional
[{'id': 6754, 'first_name': 'Bibi', 'last_name': 'Fricotin', 'birth_date': datetime.date(1924, 10, 5)}, {'id': 6755, 'first_name': 'Corto', 'last_name': 'Maltese', 'birth_date': datetime.date(1975, 1, 7)}]

You can also get a subset of the attributes by passing a list of columns names to ho_select:

>>> print(list(people.ho_select('last_name')))
[{'last_name': 'Lagaffe'}, {'last_name': 'Fricotin'}]

Note: The offset and limit still apply.

Select one: the ho_get method

The ho_get method returns an Relation object whose fields are populated with the values from the corresponding row in the database. It raises an ExpectedOneError Exception if 0 or more than 1 rows match the intention. The returned object is a singleton (see below).

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

is equivalent to

lagaffe = Person(last_name='Lagaffe')
if lagaffe.ho_is_empty() or lagaffe.ho_count() > 1:
    raise ExcpetedOneError
gaston = Person(**next(lagaffe.ho_select()))
gaston._ho_is_singleton = True

You could use ho_get to retreive the id of the row:

gaston_id = Person(last_name='Lagaffe').ho_get('id').id.value

Is it a set? Is it an element of the set?

Let's go back to our definition of the class Person. We would like to write a property that returns the full name of a person.

class Person(halftest.get_relation_class('actor.person')):
    # [...]
    @property
    def full_name(self):
        return f'{self.first_name} {self.last_name}'

As such, the full_name property wouldn't make much sense:

lagaffe = Person(last_name='Lagaffe')
lagaffe.full_name # returns 'None Lagaffe'

In this case, you can use the @singleton decorator to ensure that the self object refers to one and only one element:

from half_orm.relation import singleton

class Person(halftest.get_relation_class('actor.person')):
    @property
    @singleton
    def full_name(self):
        return f'{self.first_name} {self.last_name}'

gaston = Person(first_name='Gaston')
gaston.full_name # now returns 'Gaston Lagaffe'

If zero or more than one person in the actor.person table had Gaston as their first name, a NotASingletonError exception would be raised:

half_orm.relation_errors.NotASingletonError: Not a singleton. Got X tuples

Forcing _ho_is_singleton attribute. (advanced)

By forcing the attribute _ho_is_singleton of a Relation object to True, you can avoid some unnecessary ho_get() that a @singleton decorator would have triggered. Here is an example:

class Person(halftest.get_relation_class('actor.person')):
    # [...]
    @singleton
    def do_something_else(self):
        "Needs self to be a singleton"
        ...

    def do_something(self):
        for elt in self.ho_select():
            pers = Person(**elt)
            pers._ho_is_singleton = True # You must be pretty sure of what you're doing here. See the warning and the explanation.
            pers.do_something_else() # Warning! do_something_else won't check that pers is indeed a singleton

Warning! By setting _ho_is_singleton value to True, you disable the check that @singleton would have made before executing do_something_else. This example works for two reasons:

1. ho_select is called without argument ensuring that all columns are retreived from the database. Note: Calling ho_select with columns corresponding to the primary key as arguments would also have worked;
2. The table actor.person has a primary key which makes it a set (ie. each element returned by select is indeed a singleton).

ho_update

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

gaston = Person(first_name='Gaston')
gaston.ho_update(birth_date='1970-01-01')

Let's look at how we could turn the last name into capital letters for a subset of people:

class Person(halftest.get_relation_class('actor.person')):
    # [...]

    def upper_last_name(self):
        "tranform last name to upper case."

        def update(self):
            for d_pers in self.ho_select('id', 'last_name'):
                pers = Person(**d_pers)
                pers.ho_update(last_name=d_pers['last_name'].upper())
        with Transaction(halftest):
            update(self)

Again, we insure the atomicity of the transaction using the Transaction(halftest) context manager.

>>> a_pers = Person(last_name=('ilike', '_a%'))
>>> print([elt.last_name for elt in list(a_pers.ho_select())])
>>> a_pers = Person(last_name=('ilike', '_a%'))
>>> print([elt['last_name'] for elt in a_pers.ho_select('last_name')])
['Lagaffe', 'Maltese', 'Talon']
>>> a_pers.upper_last_name()
>>> print([elt['last_name'] for elt in a_pers.ho_select('last_name')])
['LAGAFFE', 'MALTESE', 'TALON']

Returning values

To return the updated values, you can add to ho_update the column names you want to get, or * if you want to get all the columns.

>>> gaston.ho_update('*', birth_date='1970-01-01')

Update all data in a table

If you want to update all the data in a relation, you must set the argument update_all to True. A RuntimeError is raised otherwise.

Person().ho_update(birth_date='1970-01-01', update_all=True)

ho_delete

The ho_delete method allows you to remove a set of elements from a table:

gaston = Person(first_name='Gaston')
gaston.ho_delete()

To remove every tuples from a table, you must set the argument delete_all to True. A RuntimeError is raised otherwise.

Person().ho_delete(delete_all=True)
if not Person().ho_is_empty():
    print('Weird! You should check your "on delete cascade".')

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

Returning values

As for ho_update, to return the deleted values, you can add to ho_delete the column names you want to get, or * if you want to get all the columns.

>>> gaston.ho_delete('first_name', 'last_name', 'birth_date')

Working with foreign keys [WIP]

This is a work in progress

A relational object integrates all the material necessary to process its foreign keys and the foreign keys that point to this object. When you print the object, its representation ends with the information about the foreign keys:

To use the foreign keys as direct attributes of the class, copy/paste the Fkeys below into
your code as a class attribute and replace the empty string key(s) with the alias(es) you
want to use. The aliases must be unique and different from any of the column names. Empty
string keys are ignored.

Fkeys = {
    [...]
}

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

>>> class Post(halftest.get_relation_class('blog.post')):
...     pass
...
>>> Post()

Inherits: <class '__main__.Post'>

This class allows you to manipulate the data in the PG relation:
TABLE: "halftest":"blog"."post"
DESCRIPTION:
The table blog.post contains all the post
made by a person in the blogging system.
FIELDS:
- id:                (int4) NOT NULL
- title:             (text)
- content:           (text)
- author_first_name: (text)
- author_last_name:  (text)
- author_birth_date: (date)

PRIMARY KEY (id)
UNIQUE CONSTRAINT (title, content)
FOREIGN KEYS:
- _reverse_fkey_halftest_blog_comment_post_id: ("id")
 ↳ "halftest":"blog"."comment"(post_id)
- author: ("author_first_name", "author_last_name", "author_birth_date")
 ↳ "halftest":"actor"."person"(first_name, last_name, birth_date)

To use the foreign keys as direct attributes of the class, copy/paste the Fkeys below into
your code as a class attribute and replace the empty string key(s) with the alias(es) you
want to use. The aliases must be unique and different from any of the column names. Empty
string keys are ignored.

Fkeys = {
    '': '_reverse_fkey_halftest_blog_comment_post_id',
    '': 'author',
}

It has two foreign keys named _reverse_fkey_halftest_blog_comment_post_id and author:

• author is the foreign key that refrences an actor.person from the table blog.post.
• _reverse_fkey_halftest_blog_comment_post_id is the foreign key that references a blog.post from the table blog.comment. The foreign key is traversed in opposite direction (from blog.post to blog.comment).

We add the aliases of our foreign keys by defining the class attribute Fkeys :

class Post(halftest.get_relation_class('blog.post')):
    Fkeys = {
        'comments_rfk': '_reverse_fkey_halftest_blog_comment_post_id',
        'author_fk': 'author'
    }

Note: By convention, we suffix by _fk the foreign keys and by _rfk the foreign keys traversed in reverse. The plural in comments_rfk indicates that a post can be referenced by many comments.

A foreign key is a transitional object, so when you instanciate a FKey object, you get the relation it points to. The original constraint is propagated through the foreign key.

Given a post defined by a constraint:

a_post = Post(**constraint)
comments_on_a_post = a_post.comments_rfk()
author_of_a_post = a_post.author_fk()

You can also add a filter on a foreign key.

comments_on_a_post_containing_simple = a_post.comment_rfk(content=('ilike', '%simple%'))

The Fkey class has the set method which allows you to constrain a foreign key with a Relation object. To get the comments made by Gaston, we simply constraint the author_fk Fkey to reference the entry corresponding to Gaston in the actor.person table. To do so, we use the Fkey.set() method:

gaston = Person(first_name='Gaston')
gaston_comments = Comment()
gaston_comments.author_fk.set(gaston)
print(list(gaston_comments.ho_select())

Chaining foreign keys

Important note: Foreign key chaining will only work if the modules corresponding to the tables are ordered according to the names of the tables in the package. See the hop command.

You can easily chain foreign keys. For example, if you want to get all the comments made by Gaston on his own posts:

gaston = {'last_name':'Lagaffe', 'first_name':'Gaston', 'birth_date':'1957-02-28'}
gaston_id = Person(**gaston).ho_get('id').id.value # we ensure that Gaston is a singleton
list(gaston
    .post_rfk(**gaston)
    .comment_rfk(author_id=gaston_id))

Note: the blog.post table declares a foreign key on actor.person(first_name, last_name, birth_date) while the blog.comment table declares a foreign key on actor.person(id).

The ho_join method [deprecated]

From version 0.11 onward, the ho_join method has been deprecated. It was too messy and can easily be replaced using the foreign keys.

For example, the old code:

lagaffe = Person(last_name='Lagaffe')
res = lagaffe.ho_join(
    (Comment(), 'comments', ['id', 'post_id']),
    (Post(), 'posts', 'id')
)

becomes:

res = []
lagaffe = Person(last_name='Lagaffe')
for idx, pers in enumerate(lagaffe):
    res.append(pers)
    res[idx] = {}
    posts = Person(**pers).post_rfk()
    res[idx]['posts'] = list(posts.ho_select('id'))
    res[idx]['comments'] = list(posts.comment_rfk().ho_select('id', 'post_id'))

PostgreSQL functions and stored procedures

half_orm.model.Model class provides two methods to deal with functions and stored procedures: execute_function and call_procedure. You can pass parameters as a list or a dictionary (for named parameters). The returned value of execute_function is a list of dict like objects.

from half_orm.model import Model
halftest = Model('halftest')

res = halftest.execute_function('schema.my_function', *args)
res = halftest.execute_function('schema.my_function', **kwargs) # for named parameters

half_test.call_procedure('schema.my_procedure', *args)
half_test.call_procedure('schema.my_procedure', **kwargs) # for named parameters

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.

WARING: SQL INJECTION RISK! This method calls the psycopg2 method cursor.execute. Make sure you read the psycopg2 documentation on passing parameters to SQL queries if you need to use execute_query.

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

Next: hop, the GitOps half_orm packager [WIP][alpha]

The hop command, directly provided in this package (from version 0.8.0rc1), allows you to:

• create a Python package corresponding to the model of your database;
• patch the model and the corresponding Python code;
• test your database model and your business code.

More at https://github.com/collorg/halfORM/blob/main/doc/hop.md

Want to contribute?

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