Q:

Why is "Object of type Decimal is not JSON serializable" - when using marshmallow with SQLAlchemy automap? 

Using automap_base from sqlalchemy.ext.automap to map my tables. 
Not able to shema.dumps(result);
getting
raise TypeError(f'Object of type {o.__class__.__name__} '
TypeError: Object of type Decimal is not JSON serializable

Tried using JSON custom decoders, but no use. 
from flask import Flask
from flask_sqlalchemy import SQLAlchemy
from sqlalchemy.orm import Session
from sqlalchemy.ext.automap import automap_base
from flask_marshmallow import Marshmallow

app = Flask(__name__)
app.config['SQLALCHEMY_DATABASE_URI'] = 'sqlite:///database.db'

db = SQLAlchemy(app)
ma = Marshmallow(app)

engine = db.engine
session = Session(engine)

Base = automap_base()
Base.prepare(engine, reflect=True)

MyTable = Base.classes.my_table

class MyTableSchema(ma. ModelSchema):
    class Meta:
        model = MyTable

@app.route("/")
def api():
    all_rows = session.query(MyTable).all()
    schema = MyTableSchema(many=True)
    response = schema.dumps(all_rows)

    return response

if __name__ == '__main__':
    app.run(debug=True)

A:

An easy workaround to transform an SQLAlchemy result object in JSON is using simplejson. 
You just need to import it (import simplejson) and it works. 
Using your example:
import simplejson
...
@app.route("/")
def api():
    all_rows = session.query(MyTable).all()
    response = simplejson.dumps(all_rows)

