python - MongoAlchemy文档通过Flask-MongoAlchemy编码为JSON

我想我这里少了点小东西。我正在测试python framework flask和flask mongoalchemy，并希望将实体转换为json输出。这是我的代码（摘要）：

from flask import Flask
from flaskext.mongoalchemy import MongoAlchemy

try:
    from bson.objectid import ObjectId
except:
    pass

#a bunch of code to open the mongoDB

class ClassA(db.Document):
    title = db.StringField()
    field1 = db.StringField()
    field2 = db.BoolField()

@app.route('/api/classA', methods=['GET'])
def api_list_all
    a = ClassA.query.all()
    result = []
    for b in a:
        result.append(b.wrap())
    print result
    return json.dumps(result)

如果没有json.dumps行，print语句将提示正确的结果。但只有当我对result运行json.dumps时，它才会产生：
类型错误：objectid（“…”）不是json可序列化的
我错过了什么？

最佳答案

结果是某种类型的mongo文档，其中包含objectid类型的内容，您必须告诉json如何反序列化。对于其他特定于Mongo的类型，例如referenceField（）、embeddeddocumentField（）等，您也会遇到同样的问题。
您必须编写一个反序列化函数，可以将其传递给json。我用的是：

def encode_model(obj, recursive=False):
    if obj is None:
        return obj
    if isinstance(obj, (mongoengine.Document, mongoengine.EmbeddedDocument)):
        out = dict(obj._data)
        for k,v in out.items():
            if isinstance(v, ObjectId):
                if k is None:
                    out['_id'] = str(v)
                    del(out[k])
                else:
                    # Unlikely that we'll hit this since ObjectId is always NULL key
                    out[k] = str(v)
            else:
                out[k] = encode_model(v)
    elif isinstance(obj, mongoengine.queryset.QuerySet):
        out = encode_model(list(obj))
    elif isinstance(obj, ModuleType):
        out = None
    elif isinstance(obj, groupby):
        out = [ (g,list(l)) for g,l in obj ]
    elif isinstance(obj, (list)):
        out = [encode_model(item) for item in obj]
    elif isinstance(obj, (dict)):
        out = dict([(k,encode_model(v)) for (k,v) in obj.items()])
    elif isinstance(obj, datetime.datetime):
        out = str(obj)
    elif isinstance(obj, ObjectId):
        out = {'ObjectId':str(obj)}
    elif isinstance(obj, (str, unicode)):
        out = obj
    elif isinstance(obj, float):
        out = str(obj)
    else:
        raise TypeError, "Could not JSON-encode type '%s': %s" % (type(obj), str(obj))
    return out

然后将结果处理为：

return json.dumps(result, default=encode_model)

或是有这种效果的东西。