如何在NDB中的重复字符串属性上使用列表对象进行查询 [英] How to query with a list object on repeated string property in ndb

问题描述

在使用Google App Engine时，我使用ndb来管理数据. 我有一个使用重复的字符串属性保存字符串值列表的实体. 如何使用等于属性值的列表对象进行查询?

With google app engine, I use ndb to manange my data. I have a entity that use a repeated string property to save a list of string values. How can I query with a list object that is equal to the property value?

我的意思是，实体是

class Question(ndb.Model):
    question = ndb.StringProperty(required=True)
    choices = ndb.StringProperty(repeated=True)

我想这样查询:

question = "The question"
choices = ["Choice1", "Choice2", ..., "Choice6"]
Question.query(Question.question==question, Question.choices==choices)

推荐答案

当您具有重复属性时，将无法执行查询以完全等同于该列表的查询.

When you have a repeated property, there is no way to perform a query searching for exact equality of that list.

例如，一个带有以下内容的Question实体:

For example, a Question entity with:

question = "A question"
choices = ["A", "B", "C"]

将匹配以下查询的全部

Question.query(Question.choices=="A")
Question.query(Question.choices=="B")
Question.query(Question.choices=="C")

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解决方案1:计算属性

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Solution 1: Computed Property

如果您要编写一个查询，其中整个列表相等，则可以使用计算属性，该属性是重复属性的字符串化版本:

If you want to write a query where the entire list is equal, you could use a computed property that is a stringified version of your repeated property:

class Question(ndb.Model):
    question = ndb.StringProperty(required=True)
    choices = ndb.StringProperty(repeated=True)
    choices_list = ndb.ComputedProperty(lambda self: return ",".join(self.choices))

然后使用choices_list进行查询:

choices = [...]
Question.query(Question.choices_list==",".join(choices))

当然，只有choices完全匹配(包括顺序)时，此匹配项才会出现.

Of course, this will only match if the choices match exactly (including order).

解决方案2:使用AND

Solution 2: Using AND

您还可以使用AND查询:

You could also use an AND query:

choices = [...]
Questions.query(
                ndb.AND(Question.choices==choices[0],
                ....,
                Question.choices==choices[n])) # Do this in a programmatic way

此解决方案的好处在于，列表的顺序无关紧要，但是可能会导致误报，因为对["A"，"B"]的查询将匹配具有选择项["A"的实体， "B"，"C"].

This solution has the upside that the ordering of your list does not matter, however it may result in false positives, as a query for ["A", "B"] will match an entity with choices ["A", "B", "C"].

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