how can i query data filtered by a JSON Column in SQLAlchemy?

Assuming that your table is name "custom" and your json field is named "data" the following sql statement will get your results where the value subfield is equal to "what I want".

sql = text("select * from custom where data->>'value'= 'what I want'")
result = db.engine.execute(sql)

According to the documentation, it can be done using cast:

from sqlalchemy.types import Unicode

Custom.query.filter(['value'].astext.cast(Unicode) == "what I want")