How to Check if a `sql.Null[Type]` Field is Valid in Go Templates?

Testing Valid Fields in Go Templates

In Go's database/sql package, Null[Type] structs assist in mapping database values with potential nulls into code. Determining whether a struct field is null, i.e., its Valid property is false, can be challenging.

To display a SQL field, use the .Value property:

<code class="go">{{ .MyStruct.MyField.Value }}</code>

When requiring more complex comparisons, testing the validity of the field becomes important. Using if with .Value alone results in errors if .MyField is not Valid.

Attempts to test the existence of a valid field using and and or template functions are also unsuccessful. The evaluation does not terminate early even if the result is clear from the first argument.

Instead, consider using nested if blocks:

<code class="go">{{if $.MyStruct.MyField}}
    {{if eq $.MyStruct.MyField.Value .}}selected="selected"{{end}}
{{end}}</code>

Alternatively, the with block can be employed, but it modifies the current template context:

<code class="go">{{range .SomeSlice}}
    {{with $.MyStruct.MyField}}
        {{if eq .Value .}}selected="selected"{{end}}
    {{end}}
{{/range}}</code>

Note that sql.NullXX types are non-nil structs. To determine if their Value method will return a non-nil value, check their Valid field:

<code class="go">{{if $.MyStruct.MyField.Valid}}
    {{if eq $.MyStruct.MyField.Value .}}selected="selected"{{end}}
{{end}}</code>

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