Add unique constraint, None option

datalite/__init__.py

@@ -1,3 +1,12 @@
-__all__ = ['commons', 'datalite_decorator', 'fetch', 'migrations', 'datalite']
+__all__ = ['commons', 'datalite_decorator', 'fetch', 'migrations', 'datalite', 'constraints']
+
+from dataclasses import dataclass
+
 
 from .datalite_decorator import datalite
+
+
+@dataclass
+class Student:
+    id_: int
+    name: str

datalite/commons.py

@@ -1,8 +1,14 @@
 from dataclasses import Field
 from typing import Any, Optional, Dict, List
+from .constraints import Unique
 import sqlite3 as sql
 
 
+type_table: Dict[Optional[type], str] = {None: "NULL", int: "INTEGER", float: "REAL",
+                                         str: "TEXT", bytes: "BLOB"}
+type_table.update({Unique[key]: f"{value} NOT NULL UNIQUE" for key, value in type_table.items()})
+
+
 def _convert_type(type_: Optional[type], type_overload: Dict[Optional[type], str]) -> str:
     """
     Given a Python type, return the str name of its
@@ -30,7 +36,9 @@ def _convert_sql_format(value: Any) -> str:
     >>> _convert_sql_format("John Smith")
     '"John Smith"'
     """
-    if isinstance(value, str):
+    if value is None:
+        return "NULL"
+    elif isinstance(value, str):
         return f'"{value}"'
     elif isinstance(value, bytes):
         return '"' + str(value).replace("b'", "")[:-1] + '"'
@@ -82,5 +90,5 @@ def _create_table(class_: type, cursor: sql.Cursor, type_overload: Dict[Optional
     sql_fields = "obj_id INTEGER PRIMARY KEY AUTOINCREMENT, " + sql_fields
     cursor.execute(f"CREATE TABLE IF NOT EXISTS {class_.__name__.lower()} ({sql_fields});")
 
-type_table: Dict[Optional[type], str] = {None: "NULL", int: "INTEGER", float: "REAL",
+
-                                                 str: "TEXT", bytes: "BLOB"}
+

datalite/constraints.py

@@ -0,0 +1,26 @@
+"""
+datalite.constraints module introduces constraint
+    types that can be used to hint field variables,
+    that can be used to signal datalite decorator
+    constraints in the database.
+"""
+from typing import TypeVar, Union, Tuple
+
+T = TypeVar('T')
+
+
+class ConstraintFailedError(Exception):
+    """
+    This exception is raised when a Constraint fails.
+    """
+    pass
+
+
+"""
+Dataclass fields hinted with this type signals
+    datalite that the bound column of this
+    field in the table is NOT NULL and UNIQUE.
+"""
+Unique = Union[Tuple[T], T]
+
+

datalite/datalite_decorator.py

@@ -2,10 +2,12 @@
 Defines the Datalite decorator that can be used to convert a dataclass to
 a class bound to a sqlite3 database.
 """
-
+from sqlite3.dbapi2 import IntegrityError
 from typing import Dict, Optional, List, Callable
 from dataclasses import Field, asdict
 import sqlite3 as sql
+
+from constraints import ConstraintFailedError
 from .commons import _convert_sql_format, _convert_type, _create_table, type_table
 
 
@@ -22,11 +24,14 @@ def _create_entry(self) -> None:
         table_name: str = self.__class__.__name__.lower()
         kv_pairs = [item for item in asdict(self).items()]
         kv_pairs.sort(key=lambda item: item[0])  # Sort by the name of the fields.
+        try:
             cur.execute(f"INSERT INTO {table_name}("
                         f"{', '.join(item[0] for item in kv_pairs)})"
                         f" VALUES ({', '.join(_convert_sql_format(item[1]) for item in kv_pairs)});")
             self.__setattr__("obj_id", cur.lastrowid)
             con.commit()
+        except IntegrityError:
+            raise ConstraintFailedError("A constraint has failed.")
 
 
 def _update_entry(self) -> None:

docs/constraints.rst

@@ -0,0 +1,67 @@
+Constraints
+================
+
+One of the most useful features provided by SQLite is the concept of
+*constraints*. Constraints signal the SQL engine that the values hold in a
+specific column **MUST** abide by specific constraints, these might be
+
+* Values of this column cannot be ``NULL``. (``NOT NULL``)
+* Values of this column cannot be repeated. (``UNIQUE``)
+* Values of this column must fulfill a condition. (``CHECK``)
+* Values of this column can be used to identify a record. (``PRIMARY``)
+* Values of this column has a default value. (``DEFAULT``)
+
+Some of these constraints are already implemented in datalite. With all of the set,
+is planned to be implemented in the future.
+
+Default Values
+---------------
+
+Columns can be given default values. This is done the same way you would give a
+datafield a default value.
+
+.. code-block:: python
+
+    @datalite("db.db")
+    @dataclass
+    class Student:
+        id_: int
+        name: str = "Albert Einstein"
+
+Therefore, from now on, any ``Student`` object, whose name is not specified, will
+have the default name ``"Albert Einstein"`` and if ``.create_entry()`` method is
+called on them, the newly inserted record will, by default, have this value in its
+corresponding column.
+
+Unique Values
+--------------
+
+Declaring a  field unique is done by a special ``TypeVar`` called ``Unique``
+this uniqueness check is done in the database level, this introduces has some pros,
+but also some cons.
+
+Pushing the uniqueness check to the database level introduces a better ability to
+handle concurrency for applications with large traffic, however, this check only
+runs when an item is registered, which means no problem will raise in
+the object creation *even if* another object of the same type with the same value
+hold in the unique field exists, no error will raise. However, if another *record*
+with the same value in the unique field is recorded in the bound database, upon
+the invocation of the ``.create_entry()`` will raise the ``ConstraintFailedError``
+exception.
+
+Uniqueness constraint is declared thusly:
+
+.. code-block:: python
+
+    @datalite("db.db")
+    @dataclass
+    class Student:
+        id_: Unique[int]
+        name: str = "Albert Einstein"
+
+Hinting a field with the ``Unique[T]`` type variable will introduce two rules:
+
+#.  The values in the column of the field in the table that represents the dataclass ``Student`` in the bound database ``db.db`` cannot be NULL, thus the corresponding field cannot be assigned ``None``.
+#.  These same values **must** be unique for each and every record.
+
+Failure of any of these two rules will result in a ``ConstraintFailedError`` exception.

docs/datalite.rst

@@ -6,6 +6,14 @@ datalite Module
 
 .. autodecorator:: datalite.datalite
 
+datalite.constraints module
+----------------------------------
+
+.. automodule:: datalite.constraints
+    :members:
+    :undoc-members:
+    :show-inheritence:
+
 datalite.fetch module
 ---------------------
 

docs/index.rst

@@ -19,6 +19,7 @@ Documentation
 
    installation
    decorator
+   constraints
    fetch
    migration
    datalite

docs/migration.rst

@@ -4,3 +4,16 @@ Schema Migrations
 Datalite provides a module, ``datalite.migrations`` that handles schema migrations. When a class
 definition is modified, ``datalite.migrations.basic_migration`` can be called to automatically
 transfer records to a table fitting the new definitions.
+
+Let us say we have made changes to the fields of a dataclass called ``Student`` and now,
+we want these changes to be made to the database. More specifically, we had a field called
+``studentt_id`` and realised this was a typo, we want it to be named into ``student_id``,
+and we want the values that was previously hold in this column to be persistent despite the
+name change. We can achieve this easily by:
+
+.. code-block:: python
+
+    datalite.basic_migration(Student, {'studentt_id': 'student_id'})
+
+This will make all the changes, if we had not provided the second argument,
+the values would be lost.

setup.py

@@ -5,7 +5,7 @@ with open("README.md", "r") as fh:
 
 setuptools.setup(
     name="datalite", # Replace with your own username
-    version="0.5.6",
+    version="0.5.7",
     author="Ege Ozkan",
     author_email="egeemirozkan24@gmail.com",
     description="A small package that binds dataclasses to an sqlite database",

test/main_tests.py

@@ -1,5 +1,9 @@
 import unittest
+from typing import Final
+
+from constraints import ConstraintFailedError
 from datalite import datalite
+from datalite.constraints import Unique
 from datalite.fetch import fetch_if, fetch_all, fetch_range, fetch_from, fetch_equals, fetch_where
 from sqlite3 import connect
 from dataclasses import dataclass, asdict
@@ -40,10 +44,16 @@ class Migrate1:
 @datalite(db_path='test.db')
 @dataclass
 class Migrate2:
-    cardinal: int
+    cardinal: Unique[int] = 1
     str_: str = "default"
 
 
+@datalite(db_path='test.db')
+@dataclass
+class ConstraintedClass:
+    unique_str: Unique[str]
+
+
 def getValFromDB(obj_id = 1):
     with connect('test.db') as db:
         cur = db.cursor()
@@ -163,5 +173,25 @@ class DatabaseMigration(unittest.TestCase):
         _drop_table('test.db', 'migrate1')
 
 
+def helperFunc():
+    obj = ConstraintedClass("This string is supposed to be unique.")
+    obj.create_entry()
+
+
+class DatabaseConstraints(unittest.TestCase):
+    def setUp(self) -> None:
+        self.obj = ConstraintedClass("This string is supposed to be unique.")
+        self.obj.create_entry()
+
+    def testUniquness(self):
+        self.assertRaises(ConstraintFailedError, helperFunc)
+
+    def testNullness(self):
+        self.assertRaises(ConstraintFailedError, lambda : ConstraintedClass(None).create_entry())
+
+    def tearDown(self) -> None:
+        self.obj.remove_entry()
+
+
 if __name__ == '__main__':
     unittest.main()