GIN problems with posted data validation - go

Question

I'm currently learning GO and I wanted to replicate one of the services we are running in production using GO for training. I'm working with the framework GIN and I need to validate a simple struct like this. It is posted to the search endpoint
type SearchData struct {
Field1 string `json:"field1,omitempty"`
Field2 string `json:"field2,omitempty"`
Field3 string `json:"field,omitempty" binding:"alphanum"`
}
first problem: I can't check if Field3 != nil because is a string, one solution I found by reading online is that I can cast it as a pointer, the problem is that then I get an error from the binding alphanum because nil fail the validation.
second problem: I can remove the omitempty check but if I do so the alphanum validator fail and the fields are set to their default value "". I can't check if the user want to submit "" or is the bindingJSON method that is setting the default value.
By reading online I see that there isn't a proper solution to this problem (or maybe yes), as I'm new to GO I will be happy to see how other people handle this in published / production APIs.
EDIT: This is how I parse the json
func Search(c *gin.Context) {
db := c.MustGet("db").(*mgo.Database)
var json SearchData
if err := c.ShouldBindJSON(&json); err != nil {
fmt.Println(err)
utils.ReturnError400(c, "Invalid")
return
}
// the json is used here
}

Answer 1

Gin return an empty string if the parameter was not provided.
You can use the following method in order to verify if the string is provided:
// IsBlank is delegated to verify that the does not contains only empty char
func IsBlank(str string) bool {
// Check length
if len(str) > 0 {
// Iterate string
for i := range str {
// Check about char different from whitespace
if str[i] > 32 {
return false
}
}
}
return true
}
You can read some useful utils method in this repository in order to validate string and execute common task on string:
https://github.com/alessiosavi/GoGPUtils/blob/master/string/stringutils.go
For the second problem, you can use the DefaultQuery in order to set a predefined value if no one are provided, here an example
func get() gin.HandlerFunc {
fn := func(c *gin.Context) {
// firstname will be "NOT_PRESENT" if the user does not send the parameter
firstname := c.DefaultQuery("test", "NOT_PRESENT")
lastname := c.Query("test1")
fmt.Println(firstname, lastname)
}
return gin.HandlerFunc(fn)
}
Here the documentation about DefaultQuery:
https://godoc.org/github.com/gin-gonic/gin#Context.DefaultQuery