JSON Data Type Support


Recently native JSON data type support has been added in all major database systems. JSON support introduces dynamic data structures commonly found in NoSQL databases. Usually they are used when working with highly varying data or when the exact data structure is hard to predict.

Pony allows working with JSON data stored in your database using Python syntax.

Declaring a JSON attribute

For declaring a JSON attribute with Pony you should use the Json type. This type can imported from pony.orm package:

from pony.orm import *

db = Database()

class Product(db.Entity):
    id = PrimaryKey(int, auto=True)
    name = Required(str)
    info = Required(Json)
    tags = Optional(Json)

db.bind('sqlite', ':memory:', create_db=True)

The info attribute in the Product entity is declared as Json. This allows us to have different JSON structures for different product types, and make queries to this data.

Assigning value to the JSON attribute

Usually, a JSON structure contains a combination of dictionaries and lists containing simple types, such as numbers, strings and boolean. Let’s create a couple of objects with a simple JSON structure:

p1 = Product(name='Samsung Galaxy S7 edge',
                 'display': {
                    'size': 5.5,
                 'battery': 3600,
                 '3.5mm jack': True,
                 'SD card slot': True,
                 'colors': ['Black', 'Grey', 'Gold'],
             tags=['Smartphone', 'Samsung'])

p2 = Product(name='iPhone 6s',
                 'display': {
                    'size': 4.7,
                    'resolution': [750, 1334],
                    'multi-touch': True,
                 'battery': 1810,
                 '3.5mm jack': True,
                 'colors': ['Silver', 'Gold', 'Space Gray', 'Rose Gold'],
             tags=['Smartphone', 'Apple', 'Retina'])

In Python code a JSON structure is represented with the help of the standard Python dict and list. In our example, the info attribute is assigned with a dict. The tags attribute keeps a list of tags. These attributes will be serialized to JSON and stored in the database on commit.

Reading JSON attribute

You can read a JSON attribute as any other entity attribute:

>>> Product[1].info
{'battery': 3600, '3.5mm jack': True, 'colors': ['Black', 'Grey', 'Gold'],
'display': 5.5}

Once JSON attribute is extracted from the database, it is deserialized and represented as a combination of dicts and lists. You can use the standard Python dict and list API for working with the value:

>>> Product[1].info['colors']
['Black', 'Grey', 'Gold']

>>> Product[1].info['colors'][0]

>>> 'Black' in Product[1].info['colors']

Modifying JSON attribute

For modifying the JSON attribute value, you use the standard Python list and dict API as well:

>>> Product[1].info['colors'].append('Silver')
>>> Product[1].info['colors']
['Black', 'Grey', 'Gold', 'Silver']

Now, on commit, the changes will be stored in the database. In order to track the changes made in the JSON structure, Pony uses its own dict and list implementations which inherit from the standard Python dict and list.

Below is a couple more examples of how you can modify the the JSON value.

p = Product[1]

# assigning a new value
p.info['display']['size'] = 4.7

# popping a dict value
display_size = p.info['display'].pop('size')

# removing a dict key using del
del p.info['display']

# adding a dict key
p.info['display']['resolution'] = [1440, 2560]

# removing a list item
del p.info['colors'][0]

# replacing a list item
p.info['colors'][1] = ['White']

# replacing a number of list items
p.info['colors'][1:] = ['White']

All of the actions above are regular Python operations with attributes, lists and dicts.

Querying JSON structures

Native JSON support in databases allows not only read and modify structured data, but also making queries. It is a very powerful feature - the queries use the same syntax and run in the same ACID transactional environment, in the same time offering NoSQL capabilities of a document store inside the relational database.

Pony allows selecting objects by filtering them by JSON sub-elements. To access JSON sub-element Pony constructs JSON path expression which then will be used inside a SQL query:

# products with display size greater than 5
Product.select(lambda p: p.info['display']['size'] > 5)

In order to specify values you can use parameters:

x = 2048
# products with width resolution greater or equal to x
Product.select(lambda p: p.info['display']['resolution'][0] >= x)

In MySQL, PostgreSQL, CockroachDB and SQLite it is also possible to use parameters inside JSON path expression:

index = 0
Product.select(lambda p: p.info['display']['resolution'][index] < 2000)

key = 'display'
Product.select(lambda p: p.info[key]['resolution'][index] > 1000)


Oracle does not support parameters inside JSON paths. With Oracle you can use constant keys only.

For JSON array you can calculate length:

# products with more than 2 tags
Product.select(lambda p: len(p.info['tags']) > 2)

Another query example is checking if a string key is a part of a JSON dict or array:

# products which have the resolution specified
Product.select(lambda p: 'resolution' in p.info['display'])

# products of black color
Product.select(lambda p: 'Black' in p.info['colors'])

When you compare JSON sub-element with None, it will be evaluated to True in the following cases:

  • When the sub-element contains JSON null value

  • When the sub-element does not exist

Product.select(lambda p: p.info['SD card slot'] is None)

You can test JSON sub-element for truth value:

# products with multi-touch displays
select(p for p in Product if p.info['display']['multi-touch'])

In Python, the following values are treated as false for conditionals: None, 0, False, empty string, empty dict and empty list. Pony keeps this behavior for conditions applied for JSON structures. Also, if the JSON path is not found, it will be evaluated to false.

In previous examples we used JSON structures in query conditions. But it is also possible to retrieve JSON structures or extract its parts as the query result:

select(p.info['display'] for p in Product)

When retrieving JSON structures this way, they will not be linked to entity instances. This means that modification of such JSON structures will not be saved to the database. Pony tracks JSON changes only when you select an object and modify its attributes.

MySQL and Oracle allows using wildcards in JSON path. Pony support wildcards by using special syntax:

  • […] means ‘any dictionary element’

  • [:] means ‘any list item’

Here is a query example:

select(p.info['display'][...] for p in Product)

The result of such query will be an array of JSON sub-elements. With the current situation of JSON support in databases, the wildcards can be used only in the expression part of the generator expression.

JSON Support in Databases

For storing JSON in the database Pony uses the following types:

Starting with the version 3.9 SQLite provides the JSON1 extension module. This extension improves performance when working with JSON queries, although Pony can work with JSON in SQLite even without this module.