Validator

A Go library for the validation of HTML forms on the server side. Requires user to define validation logic of form fields in a JSON file to allow potential validation sharing with the client side.

How to use the validator

Imagine you had an HTML form as described below:

<form action="/search" method="post">
  <input type="text" name="search" value="search">
  <input type="submit" value="Submit">
</form>

and we wanted to validate that the text entered in the search box was an email address and it was at least 3 characters long. To use the validator we first need to define a JSON file which follows the following structure:

[
  {
    "id": "search",
    "rules": [
      {
        "name": "min_length",
        "value": 3
      },
      {
        "name": "email"
      }
    ]
  }
]

The id field would match the input name from the HTML, and we then define a list of rules which correspond to a key within the function map in rules.go. The value variable within in rule is an optional field which can be passed into a validation function such as the one above for the min_length rule. To define your own custom rules please see the section further down.

To use the validator you should create a new validator, passing in an io.Reader with the contents of your JSON rules. Ensure that your rules JSON contains the rules for all form fields which will be validated in this service, so the FormValidator can be re used for all requests.

import "github.com/ONSdigital/go-ns/validator"

file, err := os.Open("../path/rules.json")
if err != nil {
  // handle error
}
defer file.Close()

fv := validator.New(file)

Create a struct which matches the structure of your HTML form, defining any id selectors by adding a schema tag to your struct fields.

type Form struct {
  Search string `schema:"search"`
}

Inside your request handler function you will then want to call the Validate() function and handle any errors as follows:

  var frm Form
  if err := fv.Validate(req, &frm); if err != nil {
    if _, ok := err.(ErrFormValidationFailed); !ok {
      // handle this error as you would with any normal error
    } else {
      // we now know we have field validation errors so we can work out why a
      // particular field failed validation

      fieldErrs := err.(ErrFormValidationFailed).GetFieldErrors() // returns a map of []errors
      if len(fieldErrs["search"]) > 0 {
        // we now know something with search failed validation so we can handle
        // the error appropriately, perhaps by displaying an warning message to
        // the user on screen that their input was invalid

      }
    }
  }

Customising and Extending the validator

It is possible to add custom validation to this package. This can be done by extending the RulesList map from your own code base. Imagine we had a custom rule food, which would only validate if the value of an input was equal to the word food. The json for this could look like:

[
  {
    "id": "search",
    "rules": [
      {
        "name": "min_length",
        "value": 3
      },
      {
        "name": "email"
      },
      {
        "name": "food"
      }
    ]
  }
]

To handle this new rule you would need to add a new function to the rules map, with the key "food" and its corresponding validation function, which must follow the signature:

  func(...interface{}) error

For example:

validator.RulesList["food"] = func(vars ...interface{}) error {
  var f string
  var ok bool

  if f, ok = vars[0].(string); !ok {
    return errors.New("first parameter to food must be a string")
  }

  if f != "food" {
    return validator.FieldValidationErr{errors.New("value must equal food")}
  }
  return nil
}

Make sure you always handle the interface{} to your preferred type and throw an error if the conversion fails. When performing your validation, if it fails make sure you return a FieldValidationErr rather than just an error so the validator knows that the field has genuinely failed validation.

If you need to directly extend the validator as you known there is logic which will be reused across services, then extend the rules.go file directly and, adding your function to the RulesList map, and submit a pull request.