A simple PostgreSQL to Python mapper.
Project description
A simple PostgreSQL to Python mapper [0.9.1] and its packager [0.1.0a3]
You have a PostgreSQL database at hand and you want to interact with it in Python (≥ 3.7). Half_orm
maps your tables and views to Python classes that you can easily use to manipulate your data. If you want to build or patch a model, you might be interested in the half_orm packager
.
The 'half' part of half_orm
name indicates that it only deals with the data manipulation language (DML) part of SQL. Basically the INSERT
, SELECT
, UPDATE
and DELETE
commands. This is what makes half_orm
so easy to learn an use.
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()
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())
__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) 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: ("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)
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). You can get the number of elements in a relation with the len
function, as in len(Person())
.
To define a subset, you must specify conditions on the values of the fields (columns) with tuples of the form: (comp, value)
.
The comp
value ('=
' if ommited) 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:
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-
forand
,or
,xor
andnot
. Take a look at the algebra test file.- you can also use the
==
,!=
andin
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']
You can trigger a transaction for any combination of insert, modify or delete operations using the Relation.ho_transaction
decorator.
class Person(halftest.get_relation_class('actor.person')):
# [...]
def insert_many(self, *data):
"""Insert serveral people in a single transaction."""
@self.ho_transaction
def insert(self, *data):
for d_pers in data:
self(**d_pers).ho_insert()
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 dict
s.
>>> 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)}]
>>>
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 len(lagaffe) > 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 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 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:
ho_select
is called without argument ensuring that all columns are retreived from the database. Note: Callingho_select
with columns corresponding to the primary key as arguments would also have worked;- 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."
@self.ho_transaction
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())
update(self)
Again, we insure the atomicity of the transaction using the Relation.ho_transaction
decorator.
>>> 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()
__RCLS: <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_birth_date", "author_first_name", "author_last_name")
↳ "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 anactor.person
from the tableblog.post
._reverse_fkey_halftest_blog_comment_post_id
is the foreign key that references ablog.post
from the tableblog.comment
. The foreign key is traversed in opposite direction (fromblog.post
toblog.comment
).
We redefine our class to add the aliases for our foreign keys:
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
The ho_join
method allows you to integrate the data associated with a Relation object into the result obtained by the select
method by using foreign keys of the object or by referencing the object.
Unlike the select
method (which is a generator), the ho_join
method returns a list.
It takes a list of tuples each with two or three elements:
-
a remote Relation object that must be accessible by a foreign key (direct or "reverse");
-
the name of the key under which the associated data would be stored;
-
an optional list of columns (
str[]
) or the name of a column (str
) to be retreived from the remote object.If the third argument is omitted, all columns are retreived.
For example, the following code would return the list of people named Lagaffe
with two
additional attributes (comments
and posts
):
lagaffe = Person(last_name='Lagaffe')
res = lagaffe.ho_join(
(Comment(), 'comments', ['id', 'post_id']),
(Post(), 'posts', 'id')
)
- The data associated with
comments
is a list of dictionaries whose keys are 'id' and 'post_id'. - The data associated with
posts
is a simple list of values corresponding to the 'id' column.
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.
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, to patch the model and the corresponding Python code, to test your database model and your business code.
Want to contribute?
Fork me on Github: https://github.com/collorg/halfORM
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