JSON type with nested change tracking for SQLAlchemy
sqlalchemy-json ###############
SQLAlchemy-JSON provides mutation-tracked JSON types to SQLAlchemy_:
MutableJson
is a straightforward implementation for keeping track of top-level changes to JSON objects;NestedMutableJson
is an extension of this which tracks changes even when these happen in nested objects or arrays (Python dicts
and lists
).This is essentially the SQLAlchemy mutable JSON recipe
_. We define a simple author model which list the author's name and a property handles
for various social media handles used:
.. code-block:: python
class Author(Base):
name = Column(Text)
handles = Column(MutableJson)
Or, using the declarative mapping style:
.. code-block:: python
class Category(Base):
__tablename__ = "categories"
id = mapped_column(Integer, primary_key=True)
created_at: Mapped[DateTime] = mapped_column(DateTime, default=datetime.now)
updated_at: Mapped[DateTime] = mapped_column(
DateTime, default=datetime.now, onupdate=datetime.now
)
keywords: Mapped[list[str]] = mapped_column(MutableJson)
The example below loads one of the existing authors and retrieves the mapping of social media handles. The error in the twitter handle is then corrected and committed. The change is detected by SQLAlchemy and the appropriate UPDATE
statement is generated.
.. code-block:: python
>>> author = session.query(Author).first()
>>> author.handles
{'twitter': '@JohnDoe', 'facebook': 'JohnDoe'}
>>> author.handles['twitter'] = '@JDoe'
>>> session.commit()
>>> author.handles
{'twitter': '@JDoe', 'facebook': 'JohnDoe'}
The example below defines a simple model for articles. One of the properties on this model is a mutable JSON structure called references
which includes a count of links that the article contains, grouped by domain:
.. code-block:: python
class Article(Base):
author = Column(ForeignKey('author.name'))
content = Column(Text)
references = Column(NestedMutableJson)
With this in place, an existing article is loaded and its current references inspected. Following that, the count for one of these is increased by ten, and the session is committed:
.. code-block:: python
>>> article = session.query(Article).first()
>>> article.references
{'github.com': {'edelooff/sqlalchemy-json': 4, 'zzzeek/sqlalchemy': 7}}
>>> article.references['github.com']['edelooff/sqlalchemy-json'] += 10
>>> session.commit()
>>> article.references
{'github.com': {'edelooff/sqlalchemy-json': 14, 'zzzeek/sqlalchemy': 7}}
Had the articles model used MutableJson
like in the previous example this code would have failed. This is because the top level dictionary is never altered directly. The nested mutable ensures the change happening at the lower level bubbles up to the outermost container.
By default, sqlalchemy-json uses the JSON column type provided by SQLAlchemy (specifically sqlalchemy.types.JSON
.)
If you wish to use another type (e.g. PostgreSQL's JSONB
), your database does not natively support JSON (e.g. versions of SQLite before 3.37.2/), or you wish to serialize to a format other than JSON, you'll need to provide a different backing type.
This is done by using the utility function mutable_json_type
. This type creator function accepts two parameters:
dbtype
controls the database type used. This can be an existing type provided by SQLAlchemy or SQLALchemy-utils_, or an augmented type
_ to provide serialization to any other format;nested
controls whether the created type is made mutable based on MutableDict
or NestedMutable
(defaults to False
for MutableDict
)... code-block:: python
import json
from sqlalchemy import JSON, String, TypeDecorator
from sqlalchemy.dialects.postgresql import JSONB
from sqlalchemy_json import mutable_json_type
class JsonString(TypeDecorator):
"""Enables JSON storage by encoding and decoding on the fly."""
impl = String
def process_bind_param(self, value, dialect):
return json.dumps(value)
def process_result_value(self, value, dialect):
return json.loads(value)
postgres_jsonb_mutable = mutable_json_type(dbtype=JSONB)
string_backed_nested_mutable = mutable_json_type(dbtype=JsonString, nested=True)
sqlalchemy
Here's how to setup your development environment:
.. code-block:: shell
python -m venv .venv
. .venv/bin/activate
pip install -e ".[dev]"
# run tests
pytest
MutableJson
, rather than having that support only be available in the nested variant (https://github.com/edelooff/sqlalchemy-json/pull/51)pytest
as development dependencycmp
parameter for TrackedList.sort
)sqlalchemy.types.JSON
from deprecated JSON type provided by SQLAlchemy-utils.None
to the column resulted in an error (https://github.com/edelooff/sqlalchemy-json/issues/10)JSONType
provided by SQLAlchemy-utils_ to handle backend storage;JsonObject
to MutableJson
and NestedJsonObject
to NestedMutableJson
NestedMutableJson
can now be an array
(Python list
)Initial version. This initially carried a 1.0.0 version number but has never been released on PyPI.
.. _augmented type: https://docs.sqlalchemy.org/en/13/core/custom_types.html#augmenting-existing-types .. _mutable json recipe: http://docs.sqlalchemy.org/en/latest/core/custom_types.html#marshal-json-strings .. _sqlalchemy: https://www.sqlalchemy.org/ .. _sqlalchemy-utils: https://sqlalchemy-utils.readthedocs.io/