How Errors from Server Side are Handled On Open Event Frontend

This blog article will illustrate how the various error or status codes are handled  in  Open Event Frontend, and how the appropriate response is generated corresponding to those error codes. Open Event Frontend, relies on Open Event Server for all server operations. Open Event Server exposes  a well documented JSON:API Spec Compliant REST API. The clients using the api primarily interact with it using GET, POST , PATCH and DELETE requests. And thus for each request the API returns corresponding data as response along with it’s status code.

For instance whenever the app opens, for the landing page, all the events are fetched by making a GET request to the end point v1/events. If the request is successful and events data is returned, the status code is 200 which stands for OK in the http standard set by IANA.

Fig 1: Screenshot of google chrome developer consoles’ networking tab while making a request.

Since Open Event server is compliant with JSON:API Spec, to quote it’s official documentation, “Error objects MUST be returned as an array keyed by errors in the top level of a JSON API document.” Thus whenever there is an error, or the request is unsuccessful due to a variety of reasons, the server has a predefined format to convey the information to the front end.

The process is illustrated by the reset password form on open event frontend. When a user forgets his password, he/she has the option to reset it, using his email address. Thus the form just takes in the email address of the user and makes a POST request to the reset-password API endpoint of the server.

  • Once the request is made there are 3 possibilities (check references for error code significance):
    The request is successful and a status code of 200 is returned.
  • The email address user entered doesn’t exists and no record is found in the database. 422 status code should be returned.
  • The server is down, or the request is invalid (something unexpected has occurred). In all such scenarios error code 404 should be returned.

         .post('auth/reset-password', payload)
         .then(() => {
           this.set('successMessage', this.l10n.t('Please go to the link sent to your     

           email to reset your password'));
         .catch(reason => {
           if (reason && reason.hasOwnProperty('errors') && reason.errors[0].status

               === 422) {
             this.set('errorMessage', this.l10n.t('No account is registered with this

                      email address.'));
           } else {
             this.set('errorMessage', this.l10n.t('An unexpected error occurred.'));
         .finally(()=> {
           this.set('isLoading', false);
Figure 2 : The reset password UI

Thus as mentioned in the JSON:API docs, the errors property is expected to contain the status code and error message(optional) , which ember handles via the the catch block. The catch block is executed whenever the response from the request is not successful. The contents of the response are present in the reason property. If the status of the error is 422, the corresponding message is stored inside the errorMessage property of the component which is further used to display the alert by rendering an error block on the forgot password form.

In case there is no error, the errorMessage is undefined, and the error block is not rendered at all. In case of any other unexpected error, the standard text is displayed by initialising the errorMessage property to it.


Adding a Last Modified At column in Open Event Server

This blog article will illustrate how, with the help of SQLAlchemy, a last modified at column, with complete functionality can be added to the Open Event Server database. To illustrate the process, the blog article will discuss adding the column to the sessions api. Since last modified at is a time field, and will need to be updated each time user successfully updates the session, the logic to implement will be a slightly more complex than a mere addition of a column to the table.

The first obvious step will comprise of adding the column to the database table. To achieve the same, the column will have to be added to the model for the sessions table, as well as the schema.

In app/api/schema/

class SessionSchema(Schema):
   Api schema for Session Model
   last_modified_at = fields.DateTime(dump_only=True)

And in app/models/

import pytz

class Session(db.Model):
   """Session model class"""
   __tablename__ = 'sessions'
   __versioned__ = {
       'exclude': []
   last_modified_at = db.Column(db.DateTime(timezone=True),   
   def init(self, ..., last_modified_at=None))
     #inside init method
     self.last_modified_at =

NOTE: The users for the open event organiser server will be operating in multiple time zones and hence it is important for all the times to be in sync, hence the open event database maintains all the time in UTC timezone (python’s pytz module takes care of converting user’s local time into UTC time while storing, thus unifying the timezones.) From this, it directly follows that the time needs to be timezone aware hence timezone=true is passed, while defining the column.

Next, while initialising an object of this class, the last modified time is the time of creation, and hence is set as the initial value which basically stores the current time in UTC timezone format.

Finally, the logic for updating the last modified at column every time any other value changes for a session record needs to be implemented. SQLAlchemy provides an inbuilt support for detecting update and insert events which have been used to achieve the goal. To quote the official SQLAlchemy Docs,  “SQLAlchemy includes an event API which publishes a wide variety of hooks into the internals of both SQLAlchemy Core and ORM.

@event.listens_for(Session, 'after_update')
def receive_after_update(mapper, connection, target):
  target.last_modified_at =

The listens_for() decorator is used to register the event according to the arguments passed to it. In our case, it will register any event on the Session API (sessions table), whenever it updates.

The corresponding function defined below the decorator, receive_after_update(mapper, connection, target) is then called, and session model (table) is the the registered target with the event. It sets the value of the last_modified_at to the current time in the UTC timezone as expected.

Lastly, since the changes have been made to the database schema, the migration file needs to be generated, and the database will be upgraded to alter the structure.

The sequence of steps to be followed on the CLI will be

> python db migrate
> python db upgrade


Deployment terms in Open Event Frontend

In Open Event Frontend, once a pull request is opened, we see some tests running on for the specific pull request like ‘Codacy’, ‘Codecov’, ‘Travis’, etc. New contributors eventually get confused what the tests are about. So this blog would be a walkthrough to these terms that we use and what they mean about the PR.

Travis: Everytime you make a pull request, you will see this test running and in some time giving the output whether the test passed or failed. Travis is the continuous integration test that we are using to test that the changes that the pull request you proposed does not break any other things. Sometimes you will see the following message which indicates that your changes is breaking something else which is not intended.

Thus, by looking at the Travis logs, you can see where the changes proposed in the pull request are breaking things. Thus, you can go ahead and correct the code and push again to run the Travis build until it passes.

Codacy: Codacy is being used to check the code style, duplication, complexity and coverage, etc. When you create a pull request or update the pull request, this test runs which checks whether the code followed certain style guide or if there is duplication in code, etc. For instance let’s say if your code has a html page in which a tag has an attribute which is left undefined. Then codacy will be throwing error failing the tests. Thus you need to see the logs and go correct the bug in code. The following message shows that the codacy test has passed.


Codecov is a code coverage test which indicates how much of the code change that is proposed in the pull request is actually executed. Consider out of the 100 lines of code that you wrote, only 80 lines is being actually executed and rest is not, then the code coverage decreases. The following indicates the codecov report.

Thus, it can be seen that which files are affected by what percent.


The surge link is nothing but the deployment link of the changes in your pull request.

Thus, checking the link manually, we can test the behavior of the app in terms of UI/UX or the other features that the pull request adds.




How the Form Mixin Enhances Validations in Open Event Frontend

This blog article will illustrate how the various validations come together in  Open Event Frontend, in a standard format, strongly reducing the code redundancy in declaring validations. Open Event Frontend, offers high flexibility in terms of validation options, and all are stored in a convenient object notation format as follows:

getValidationRules() {
return {
  inline : true,
  delay  : false,
  on     : 'blur',
  fields : {
    identification: {
      identifier : 'email',
      rules      : [
          type   : 'empty',
          prompt : this.l10n.t('Please enter your email ID')
          type   : 'email',
          prompt : this.l10n.t('Please enter a valid email ID')
    password: {
      identifier : 'password',
      rules      : [
          type   : 'empty',
          prompt : this.l10n.t('Please enter your password')

Thus the validations of a form are stored as objects, where the  identifier attribute determines which field to apply the validation conditions to. The rules array contains all the rules to be applied to the determined object. Within rules, the type represents the kind of validation, whereas the prompt attribute determines what message shall be displayed in case there is a violation of the validation rule.


NOTE: In case an identifier is not specified, then the name of the rule object is itself considered as the identifier.

These validations are in turn implemented by the FormMixin. The various relevant sections of the mixin will be discussed in detail. Please check references for complete source code of the mixin.

getForm() {
return this.get($form);

The getForm function returns the calling component’s entire form object on which the validations are to be applied.

onValid(callback) {
this.getForm().form('validate form');
if (this.getForm().form('is valid')) {

The onValid function serves as the boundary level function, and is used to specify what should happen when the validation is successful. It expects a function object as i’s argument. It makes use of the getForm() function to retrieve the form and using semantic UI, determines if all the validations have been successful. In case they have, it calls the callback method passed down to it as the argument.

Next transitioning into the actual implementation, certain behavioral traits of the validations are controlled by the two global variables autoScrollToErrors and autoScrollSpeed. The former is a boolean, which is set to true if the application is designed in such a way that once the user presses the submit button and a validation fails, the browser scrolls to the first field whose validation failed with a speed specified by the latter.

Next, comes the heart of the mixin. Since it needs to be continuously determined if a field’s validation has failed, the entire logic of checking is placed inside a debounce call which is supported by ember to continuously execute a function with a gap of a certain specified time (400 ms in this case). Since the validity can be checked only after all the fields have rendered, the debounce call is placed inside a didRender call. The didRender call ensures that any logic inside it is executed only after all the elements of the relevant component have been rendered and are a part of the DOM.

didRender() {
. . .
debounce(this, () => {
  const defaultFormRules = {
    onFailure: formErrors => {
      if (this.autoScrollToErrors) {
        // Scroll to the first error message
        if (formErrors.length > 0) {
            scrollTop: this.$(`div:contains('${formErrors[0]}')`).offset().top
          }, this.autoScrollSpeed);
}, 400); . . .}


onFailure is an ES6 syntax function, which takes all the form errors as its arguments. And in case the autoScrollToErrors global variable is set to true, it checks if there are any errors or the length of the formErrors array is more than 0. In case there are errors, it makes a call to animate callback, and scrolls to the very first field which has an error with the speed defined by the previously discussed global autoScrollSpeed . The fact that the very first field is selected for scrolling off to, is ensured by the index 0 specified (formErrors[0]). Thus role of the mixin is to continuously check for any validations and in case there are, scroll to them.