在解组MongoDB文档时如何忽略空值? [英] How to ignore nulls while unmarshalling a MongoDB document?
问题描述
我想知道是否有任何方法可以让我在将MongoDB文档解组为Go结构时忽略空类型.
现在我有一些自动生成的Go结构，如下所示:
type User struct {
Name string `bson:"name"`
Email string `bson:"email"`
}
更改此结构中声明的类型不是一种选择，这就是问题所在；在我没有完全控制权的MongoDB数据库中，某些文档原本是用空值插入的，但我本来并不希望使用空值.像这样:
{
"name": "John Doe",
"email": null
}
由于在我的结构体中声明的字符串类型不是指针，它们不能接收nil值，因此，每当我尝试在我的结构体中解组该文档时，它都会返回错误.
防止这种类型的文档插入数据库将是理想的解决方案，但是对于我的用例，忽略空值也是可以接受的.因此，在将文档解组后，我的User实例将如下所示:
User {
Name: "John Doe",
Email: "",
}
我正在尝试查找某些注释标志或可以传递给方法Find/FindOne的选项，甚至可能是查询参数，以防止从数据库返回任何包含空值的字段.到目前为止没有任何成功.
mongo-go-driver中是否有针对此问题的内置解决方案?
问题是当前的bson编解码器不支持将string编码/解码为null.
一种解决方法是为string类型创建一个自定义解码器，在其中处理null值:我们只使用空字符串(更重要的是不要报告错误).
自定义解码器的类型为 bsoncodec.ValueDecoder .可以使用 bsoncodec.Registry 进行注册.例如， bsoncodec.RegistryBuilder .
可以在多个级别设置/应用注册表，甚至可以应用于整个 mongo.Client 或 mongo.Database 或仅获取 mongo.Collection ，作为他们选择的一部分，例如 options.ClientOptions.SetRegistry() .
首先让我们看看如何为string做到这一点，然后我们将了解如何改进/归纳任何类型的解决方案.
1.处理null字符串
首先，让我们创建一个自定义字符串解码器，该解码器可以将null转换为(n个空)字符串:
import (
"go.mongodb.org/mongo-driver/bson/bsoncodec"
"go.mongodb.org/mongo-driver/bson/bsonrw"
"go.mongodb.org/mongo-driver/bson/bsontype"
)
type nullawareStrDecoder struct{}
func (nullawareStrDecoder) DecodeValue(dctx bsoncodec.DecodeContext, vr bsonrw.ValueReader, val reflect.Value) error {
if !val.CanSet() || val.Kind() != reflect.String {
return errors.New("bad type or not settable")
}
var str string
var err error
switch vr.Type() {
case bsontype.String:
if str, err = vr.ReadString(); err != nil {
return err
}
case bsontype.Null: // THIS IS THE MISSING PIECE TO HANDLE NULL!
if err = vr.ReadNull(); err != nil {
return err
}
default:
return fmt.Errorf("cannot decode %v into a string type", vr.Type())
}
val.SetString(str)
return nil
}
好的，现在让我们看看如何将此自定义字符串解码器用于mongo.Client:
clientOpts := options.Client().
ApplyURI("mongodb://localhost:27017/").
SetRegistry(
bson.NewRegistryBuilder().
RegisterDecoder(reflect.TypeOf(""), nullawareStrDecoder{}).
Build(),
)
client, err := mongo.Connect(ctx, clientOpts)
从现在开始，使用此client，每当将结果解码为string值时，都会调用此注册的nullawareStrDecoder解码器来处理转换，该转换接受bson null值并设置Go空字符串"".
但是我们可以做得更好...继续阅读...
2.处理任何类型的null值:类型无关"的可感知空值的解码器
一种方法是创建一个单独的自定义解码器，并为我们希望处理的每种类型注册它.这似乎是很多工作.
我们可能(并且应该)做的是创建一个仅处理类型null的类型中立"的自定义解码器，并且如果BSON值不是null，则应调用默认解码器来处理非null值.
这非常简单:
type nullawareDecoder struct {
defDecoder bsoncodec.ValueDecoder
zeroValue reflect.Value
}
func (d *nullawareDecoder) DecodeValue(dctx bsoncodec.DecodeContext, vr bsonrw.ValueReader, val reflect.Value) error {
if vr.Type() != bsontype.Null {
return d.defDecoder.DecodeValue(dctx, vr, val)
}
if !val.CanSet() {
return errors.New("value not settable")
}
if err := vr.ReadNull(); err != nil {
return err
}
// Set the zero value of val's type:
val.Set(d.zeroValue)
return nil
}
我们只需要弄清楚nullawareDecoder.defDecoder使用什么.为此，我们可以使用默认注册表: bson.DefaultRegistry ，我们可能会针对每种类型查找默认的解码器.很好.
因此，我们现在要做的是为要处理null的所有类型注册nullawareDecoder的值.没那么难.我们只列出我们想要的类型(或这些类型的值)，我们可以通过一个简单的循环来处理所有事情:
customValues := []interface{}{
"", // string
int(0), // int
int32(0), // int32
}
rb := bson.NewRegistryBuilder()
for _, v := range customValues {
t := reflect.TypeOf(v)
defDecoder, err := bson.DefaultRegistry.LookupDecoder(t)
if err != nil {
panic(err)
}
rb.RegisterDecoder(t, &nullawareDecoder{defDecoder, reflect.Zero(t)})
}
clientOpts := options.Client().
ApplyURI("mongodb://localhost:27017/").
SetRegistry(rb.Build())
client, err := mongo.Connect(ctx, clientOpts)
在上面的示例中，我为string，int和int32注册了空感知解码器，但是您可以根据自己的喜好扩展此列表，只需将所需类型的值添加到上面的customValues切片中
I would like to know if there's any approach that would allow me to ignore null types while unmarshalling a MongoDB document into a Go struct.
Right now I have some auto-generate Go structs, something like this:
type User struct {
Name string `bson:"name"`
Email string `bson:"email"`
}
Changing the types declared in this struct is not an option, and here's the problem; in a MongoDB database, which I do not have total control, some of the documents have been inserted with null values were originally I was not expecting nulls. Something like this:
{
"name": "John Doe",
"email": null
}
As the string types declared inside my struct are not pointers, they can't receive a nil value, so whenever I try to unmarshall this document in my struct, it returns an error.
Preventing the insertion of this kind of document into the database would be the ideal solution, but for my use case, ignoring the null values would also be acceptable. So after unmarshalling the document my User instance would look like this
User {
Name: "John Doe",
Email: "",
}
I'm trying to find, either some annotation flag, or an option that could be passed to the method Find/FindOne, or maybe even a query parameter to prevent returning any field containing null values from the database. Without any success until now.
Are there any built-in solutions in the mongo-go-driver for this problem?
The problem is that the current bson codecs do not support encoding / decoding string into / from null.
One way to handle this is to create a custom decoder for string type in which we handle null values: we just use the empty string (and more importantly don't report error).
Custom decoders are described by the type bsoncodec.ValueDecoder. They can be registered at a bsoncodec.Registry, using a bsoncodec.RegistryBuilder for example.
Registries can be set / applied at multiple levels, even to a whole mongo.Client, or to a mongo.Database or just to a mongo.Collection, when acquiring them, as part of their options, e.g. options.ClientOptions.SetRegistry().
First let's see how we can do this for string, and next we'll see how to improve / generalize the solution to any type.
1. Handling null strings
First things first, let's create a custom string decoder that can turn a null into a(n empty) string:
import (
"go.mongodb.org/mongo-driver/bson/bsoncodec"
"go.mongodb.org/mongo-driver/bson/bsonrw"
"go.mongodb.org/mongo-driver/bson/bsontype"
)
type nullawareStrDecoder struct{}
func (nullawareStrDecoder) DecodeValue(dctx bsoncodec.DecodeContext, vr bsonrw.ValueReader, val reflect.Value) error {
if !val.CanSet() || val.Kind() != reflect.String {
return errors.New("bad type or not settable")
}
var str string
var err error
switch vr.Type() {
case bsontype.String:
if str, err = vr.ReadString(); err != nil {
return err
}
case bsontype.Null: // THIS IS THE MISSING PIECE TO HANDLE NULL!
if err = vr.ReadNull(); err != nil {
return err
}
default:
return fmt.Errorf("cannot decode %v into a string type", vr.Type())
}
val.SetString(str)
return nil
}
OK, and now let's see how to utilize this custom string decoder to a mongo.Client:
clientOpts := options.Client().
ApplyURI("mongodb://localhost:27017/").
SetRegistry(
bson.NewRegistryBuilder().
RegisterDecoder(reflect.TypeOf(""), nullawareStrDecoder{}).
Build(),
)
client, err := mongo.Connect(ctx, clientOpts)
From now on, using this client, whenever you decode results into string values, this registered nullawareStrDecoder decoder will be called to handle the conversion, which accepts bson null values and sets the Go empty string "".
But we can do better... Read on...
2. Handling null values of any type: "type-neutral" null-aware decoder
One way would be to create a separate, custom decoder and register it for each type we wish to handle. That seems to be a lot of work.
What we may (and should) do instead is create a single, "type-neutral" custom decoder which handles just nulls, and if the BSON value is not null, should call the default decoder to handle the non-null value.
This is surprisingly simple:
type nullawareDecoder struct {
defDecoder bsoncodec.ValueDecoder
zeroValue reflect.Value
}
func (d *nullawareDecoder) DecodeValue(dctx bsoncodec.DecodeContext, vr bsonrw.ValueReader, val reflect.Value) error {
if vr.Type() != bsontype.Null {
return d.defDecoder.DecodeValue(dctx, vr, val)
}
if !val.CanSet() {
return errors.New("value not settable")
}
if err := vr.ReadNull(); err != nil {
return err
}
// Set the zero value of val's type:
val.Set(d.zeroValue)
return nil
}
We just have to figure out what to use for nullawareDecoder.defDecoder. For this we may use the default registry: bson.DefaultRegistry, we may lookup the default decoder for individual types. Cool.
So what we do now is register a value of our nullawareDecoder for all types we want to handle nulls for. It's not that hard. We just list the types (or values of those types) we want this for, and we can take care of all with a simple loop:
customValues := []interface{}{
"", // string
int(0), // int
int32(0), // int32
}
rb := bson.NewRegistryBuilder()
for _, v := range customValues {
t := reflect.TypeOf(v)
defDecoder, err := bson.DefaultRegistry.LookupDecoder(t)
if err != nil {
panic(err)
}
rb.RegisterDecoder(t, &nullawareDecoder{defDecoder, reflect.Zero(t)})
}
clientOpts := options.Client().
ApplyURI("mongodb://localhost:27017/").
SetRegistry(rb.Build())
client, err := mongo.Connect(ctx, clientOpts)
In the example above I registered null-aware decoders for string, int and int32, but you may extend this list to your liking, just add values of the desired types to the customValues slice above.
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