Golang模板并测试有效字段 [英] Golang template and testing for Valid fields

问题描述

在Go的数据库/sql程序包中，有一堆Null [Type]结构可帮助将数据库值(及其可能的null)映射到代码中.我试图弄清楚如何测试结构 field 是否为空，或者换句话说，当其Valid属性为false时.

In Go's database/sql package, there are a bunch of Null[Type] structs that help map database values (and their possible nulls) into code. I'm trying to figure out how to test whether a struct field is null, or in other words, when its Valid property is false.

建议的打印SQL字段的方法是使用.Value属性，如下所示:

The recommended way to print a SQL field is to use the .Value property, like this:

<div>{{ .MyStruct.MyField.Value }}</div>

这很好.

但是假设我有一些更复杂的东西，例如，我需要在其他地方测试该值，例如:

But suppose I have something slightly more complicated, where I need to test the value against something else, for example:

<select name="y">
   {{ range .SomeSlice }}
       <option value="{{ . }}" {{ if eq $.MyStruct.MyField.Value .}}selected="selected"{{ end }}>{{ . }}</option>
   {{ end }}
</select>

碰巧的是，这也很好用，除非.MyField无效，在这种情况下，我会收到错误消息:错误调用eq:用于比较的无效类型".该错误是有道理的，因为Go无法将nil Field与另一个值(或类似值)进行比较.

As it happens, this works great, too, unless .MyField is not Valid, in which case I get the error, "error calling eq: invalid type for comparison". The error makes sense, because Go can't compare a nil Field against another value (or something like that).

我本以为简单"的解决方案是先测试Value是否为nil，然后将其与所需的值进行比较，如下所示:

I would have thought the 'easy' solution would be to test first whether the Value is nil, and then compare it against what I need, like this:

<select name="y">
   {{ range .SomeSlice }}
       <option value="{{ . }}" {{ if and ($.MyStruct.MyField) (eq $.MyStruct.MyField.Value .)}}selected="selected"{{ end }}>{{ . }}</option>
   {{ end }}
</select>

在这种情况下，我得到相同的调用eq:进行比较的无效类型时出错".我猜这意味着.MyField存在"，即使.MyField的值无效.所以，然后我尝试了六个其他版本，大多数都存在相同的错误，例如:

In this case, I get the same "error calling eq: invalid type for comparison". I guess that means .MyField "exists" even though the value of .MyField is not Valid. So, then I tried a half dozen other versions, mostly with the same error, for example:

<select name="y">
   {{ range .SomeSlice }}
       <option value="{{ . }}" {{ if and ($.MyStruct.MyField.Valid) (eq $.MyStruct.MyField.Value .)}}selected="selected"{{ end }}>{{ . }}</option>
   {{ end }}
</select>

至此，我意识到我真的根本不了解如何测试有效字段的存在.非常感谢您的帮助.

At this point, I'm realizing I really don't understand how to test for the existence of a valid field at all. I'd appreciate any help you might have.

谢谢.

推荐答案

Go模板中的and函数不是短路求值的(不同于Go中的&&运算符)，它的所有参数总是被求值的.引用 text/template 包doc:

The and function in Go templates is not short-circuit evaluated (unlike the && operator in Go), all its arguments are evaluated always. Quoting from text/template package doc:

and
    Returns the boolean AND of its arguments by returning the
    first empty argument or the last argument, that is,
    "and x y" behaves as "if x then y else x". All the
    arguments are evaluated.

这意味着您的{{if}}操作:

{{ if and ($.MyStruct.MyField) (eq $.MyStruct.MyField.Value .)}}

即使$.MyStruct.MyField是nil，即使条件将被评估为false，但也会评估eq $.MyStruct.MyField.Value .并导致错误.

Even though the condition would be evaluated to false if $.MyStruct.MyField is nil, but eq $.MyStruct.MyField.Value . will also be evaluated and result in the error you get.

相反，您可以嵌入多个{{if}}操作，如下所示:

Instead you may embed multiple {{if}} actions, like this:

{{if $.MyStruct.MyField}}
    {{if eq $.MyStruct.MyField.Value .}}selected="selected"{{end}}
{{end}}

您也可以使用{{with}}动作，但这也会设置点，因此您必须小心:

You may also use the {{with}} action, but that also sets the dot, so you have to be careful:

<select name="y">
   {{range $idx, $e := .SomeSlice}}
       <option value="{{.}}" {{with $.MyStruct.MyField}}
               {{if eq .Value $e}}selected="selected"{{end}}
           {{end}}>{{.}}</option>
   {{end}}
</select>

注意:

您在谈论问题中的nil值，但是sql.NullXX类型是不能为nil的结构.在这种情况下，您必须检查其Valid字段以判断其Value()方法在调用时是否将为您返回非nil值.看起来可能像这样:

You were talking about nil values in your question, but the sql.NullXX types are structs which cannot be nil. In which case you have to check its Valid field to tell if its Value() method will return you a non-nil value when called. It could look like this:

{{if $.MyStruct.MyField.Valid}}
    {{if eq $.MyStruct.MyField.Value .}}selected="selected"{{end}}
{{end}}

这篇关于Golang模板并测试有效字段的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！