# 鉴频器（discriminators）
> 原文：[The model.discriminator() function](http://mongoosejs.com/docs/discriminators.html)
## The `model.discriminator()` function
鉴别器是一个模式继承机制。他们使你重叠模式上同一标的MongoDB集合有多个模型。
假设你想在一个集合中跟踪不同类型的事件。每一件事件都会有一个时间戳，但事件表示点击链接应该有一个URL。你可以使用`model.discriminator()`函数。这个函数需要2个参数，一个模型的名字和一个鉴频器模式。它返回一个模型是基础模式的结合和鉴频器模式。
```
var options = {discriminatorKey: 'kind'};
var eventSchema = new mongoose.Schema({time: Date}, options);
var Event = mongoose.model('Event', eventSchema);
// ClickedLinkEvent is a special type of Event that has
// a URL.
var ClickedLinkEvent = Event.discriminator('ClickedLink',
new mongoose.Schema({url: String}, options));
// When you create a generic event, it can't have a URL field...
var genericEvent = new Event({time: Date.now(), url: 'google.com'});
assert.ok(!genericEvent.url);
// But a ClickedLinkEvent can
var clickedEvent =
new ClickedLinkEvent({time: Date.now(), url: 'google.com'});
assert.ok(clickedEvent.url);
```
### 鉴别器保存事件模型的集合
假设你创建一个鉴别器跟踪事件，新用户注册。这些`SignedUpEvent`实例将被存储在相同的集合作为通用的事件和`ClickedLinkEvent`实例。
```
var event1 = new Event({time: Date.now()});
var event2 = new ClickedLinkEvent({time: Date.now(), url: 'google.com'});
var event3 = new SignedUpEvent({time: Date.now(), user: 'testuser'});
var save = function (doc, callback) {
doc.save(function (error, doc) {
callback(error, doc);
});
};
async.map([event1, event2, event3], save, function (error) {
Event.count({}, function (error, count) {
assert.equal(count, 3);
});
});
```
### 鉴别器的键
mongoose讲述不同的鉴别模型之间的差异是由“鉴频器的键”，默认是`__t`。Mongoose添加一个叫做`__t`字符串路径到你的模式中，它采用追踪鉴别本文档实例。
```
var event1 = new Event({time: Date.now()});
var event2 = new ClickedLinkEvent({time: Date.now(), url: 'google.com'});
var event3 = new SignedUpEvent({time: Date.now(), user: 'testuser'});
assert.ok(!event1.__t);
assert.equal(event2.__t, 'ClickedLink');
assert.equal(event3.__t, 'SignedUp');
```
### 鉴别器添加鉴别键查询
鉴别器模型是特殊的；他们重视鉴别键查询。换句话说，`find(), count(), aggregate(),`等等，有足够的智慧来解释鉴别器。
```
var event1 = new Event({time: Date.now()});
var event2 = new ClickedLinkEvent({time: Date.now(), url: 'google.com'});
var event3 = new SignedUpEvent({time: Date.now(), user: 'testuser'});
var save = function (doc, callback) {
doc.save(function (error, doc) {
callback(error, doc);
});
};
async.map([event1, event2, event3], save, function (error) {
ClickedLinkEvent.find({}, function (error, docs) {
assert.equal(docs.length, 1);
assert.equal(docs[0]._id.toString(), event2._id.toString());
assert.equal(docs[0].url, 'google.com');
});
});
```
### 鉴别器复制的前置和后置钩子
作者也使用他们的基础模式的前置和后置的中间件。然而，你也可以把中间件来鉴别模式不影响基础模式。
```
var options = {discriminatorKey: 'kind'};
var eventSchema = new mongoose.Schema({time: Date}, options);
var eventSchemaCalls = 0;
eventSchema.pre('validate', function (next) {
++eventSchemaCalls;
next();
});
var Event = mongoose.model('GenericEvent', eventSchema);
var clickedLinkSchema = new mongoose.Schema({url: String}, options);
var clickedSchemaCalls = 0;
clickedLinkSchema.pre('validate', function (next) {
++clickedSchemaCalls;
next();
});
var ClickedLinkEvent = Event.discriminator('ClickedLinkEvent',
clickedLinkSchema);
var event1 = new ClickedLinkEvent();
event1.validate(function () {
assert.equal(eventSchemaCalls, 1);
assert.equal(clickedSchemaCalls, 1);
var generic = new Event();
generic.validate(function () {
assert.equal(eventSchemaCalls, 2);
assert.equal(clickedSchemaCalls, 1);
});
});
```
### 处理自定义\_id字段
鉴别器的字段是基础模式的字段和鉴别器模式的字段的结合，并且鉴频器模式的字段优先。这种行为变得古怪当你有一个自定义`_id`字段。一个模式默认情况下得有`_id`字段，所以基础模式的`_id`字段将得到由鉴别器模式的默认`_id`字段覆盖。
你可以通过设置`_id`选项为false 在鉴别器的模式如下图所示。
```
var options = {discriminatorKey: 'kind'};
// Base schema has a String _id...
var eventSchema = new mongoose.Schema({_id: String, time: Date},
options);
var Event = mongoose.model('BaseEvent', eventSchema);
var clickedLinkSchema = new mongoose.Schema({url: String}, options);
var ClickedLinkEvent = Event.discriminator('ChildEventBad',
clickedLinkSchema);
var event1 = new ClickedLinkEvent();
// Woops, clickedLinkSchema overwrote the custom _id
assert.ok(event1._id instanceof mongoose.Types.ObjectId);
// But if you set `_id` option to false...
clickedLinkSchema = new mongoose.Schema({url: String},
{discriminatorKey: 'kind', _id: false});
ClickedLinkEvent = Event.discriminator('ChildEventGood',
clickedLinkSchema);
// The custom _id from the base schema comes through
var event2 = new ClickedLinkEvent({_id: 'test'});
assert.ok(event2._id.toString() === event2._id);
```
