Compiling JSON as TEXT for SQLite with SQLAlchemy

A project I’m working on stores JSON objects in Postgres using its native JSON datatype. For developing the frontend of the site, running Postgres locally is unwanted, but some components still require a database to exist. To deal with this, we’re using SQLite for any local development that doesn’t modify the schema.

The problem caused by this setup is that SQLite doesn’t natively support the JSON datatype. Yes, it has the json1 extension, but SQLAlchemy’s types.JSON doesn’t work with it. To solve this, I’ve decided to simply serialize the JSON and store it as TEXT when SQLite is used.

The following code snippet shows how this works.

import sqlalchemy.types as types
import json

class StringyJSON(types.TypeDecorator):
    """Stores and retrieves JSON as TEXT."""

    impl = types.TEXT

    def process_bind_param(self, value, dialect):
        if value is not None:
            value = json.dumps(value)
        return value

    def process_result_value(self, value, dialect):
        if value is not None:
            value = json.loads(value)
        return value

# TypeEngine.with_variant says "use StringyJSON instead when
# connecting to 'sqlite'"
MagicJSON = types.JSON().with_variant(StringyJSON, 'sqlite')

Now MagicJSON is the datatype you specify when you’re defining your columns. It’s important to note that this new type is immutable, but that is fine for my specific use case. For more information about this see the Custom Types documentation for SQLAlchemy; there’s also instructions on how to make it mutable in there.

Some nice future work would be to create an sqlite.JSON type that implements SQLite’s JSON support in json1 and works with types.JSON. Hopefully I’ll have some time to look into this over the coming weeks.

Published on October 08, 2016.