Add ability to retrieve tag names from struct

[dominicbarnes]

I would like to build a SQL statement without using * using an empty struct. It would be nice if I could retrieve all the "tag names", similar to how Names() works. For example:

type Person struct {
  FirstName string `structs:"first_name"`
  LastName string `structs:"last_name"`
}

p := new(Person)

names := structs.Names(p)
// names = []string{"FirstName", "LastName"}

// not currently possible
tagNames := structs.TagNames(p)
// tagNames = []string{"first_name", "last_name"}

I would argue that this is acceptable in this library since it deals with a tag that this package manages. A temporary workaround is to export the struct to a map and extract the keys from that. (but it would be great if this library could remove that boilerplate)

[fatih]

Hi @dominicbarnes

To get the tags you also need to define the tag key name. Such as structs, json, etc.. You can do it already with the following (for your example):

package main

import (
	"fmt"

	"github.com/fatih/structs"
)

type Person struct {
	FirstName string `structs:"first_name"`
	LastName  string `structs:"last_name"`
}

func main() {
	var tagNames []string
	for _, f := range structs.New(Person{}).Fields() {
		tagNames = append(tagNames, f.Tag("structs"))
	}

	fmt.Println(tagNames)
}

As you see it's just three lines to collect all tag names. I don't want to further expand this library as I believe it already provides the necessary API. Thanks for you feedback.

[dominicbarnes]

The example you have misses quite a few details, such as additional parts in the tag like omitempty or omitnested and using - or empty string to ignore a field. Plus, it's inconvenient to establish my own for loop to accomplish this.

Would you at least be open to a PR that adds this feature? (rather than needing to write it yourself?)

[fatih]

I'm not sure those are important. The struct pkg will take care of it already. Can you share an example what you mean?

[dominicbarnes]

To elaborate on my first example:

type Person struct {
  FirstName string    `structs:"first_name"`
  LastName  string    `structs:"last_name"`
  Birthdate time.Time `structs:"birthdate,omitnested"`
  CacheKey  string    `structs:"-"`
}

Using the code you have above, I'd end up with:

[]string{"first_name", "last_name", "birthdate,omitnested", "-"}

But what I really want is:

[]string{"first_name", "last_name", "birthdate"}

[fatih]

Hi @dominicbarnes

I see what you mean. I worked on a separate, improved and powerful tags package that deals with this any many other things. Let keep this open, I'll add the necessary feature once I have time (just need to replace the internal tags.go file with the package I wrote and expose it with some well defined API's)