The curious cases of json_extract

json_extract is a function for extract data from a JSON document both in MySQL and MariaDB. The function normally works fine, for example set @j = '{"num": 42, "list": [1, 2, 3], "obj": {"name": "Edward Stark"}}'; select json_extract(@j, '$.num') as num, json_extract(@j, '$.list') as list, json_extract(@j, '$.obj') as obj; +------+-----------+--------------------------+ | num | list | obj | +------+-----------+--------------------------+ | 42 | [1, 2, 3] | {"name": "Edward Stark"} | +------+-----------+--------------------------+ But when it comes to a single JSON string, the results of json_extract is not always as expected. Continue reading

Contextvars and Thread local

Here in the post, I will share some examples about the contextvars (new in Python 3.7) and thread local. Default Value In the module level, use ContextVar.set or directly setattr for a thread local variable, won’t successfully set a default value, the value set won’t take effect in another thread. To ensure a default value, for contextvars import contextvars context_var = contextvars.ContextVar("context_var", default=0) for thread local, a sub class of thread. Continue reading

Mongodb(via MongoEngine) join query with aggregate

Since Mongodb 3.2 and MongoEngine 0.9, we can use $aggregate command to perform join queries on multiple collections in a database. This post would be a simple tutorial for join queries on Mongodb(via MongoEngine in Python) with examples. Models Setup Let’s consider models defined as below: import random import mongoengine class User(mongoengine.Document): meta = {"indexes": ['rnd']} name = mongoengine.StringField() rnd = mongoengine.FloatField(default=random.random) class Group(mongoengine.Document): meta = {"indexes": ['rnd']} name = mongoengine. Continue reading