Resolved: Flask Mongoengine how to check unique field case insensitive

from flask_mongoengine import MongoEngine

db = MongoEngine()

class User(db.Document):
    email = db.EmailField(
        unique = True,
        required = True
    )
    username = db.StringField(
        unique = True,
        required = True
    )
    password = db.StringField(
        required = True
    )
    
    def to_json(self):
        return {
            'email': self.email,
            'username': self.username,
            'password': self.password
        }

from flask import Blueprint, render_template, request, flash, redirect
from werkzeug.security import generate_password_hash
from models import User, db
import re

auth = Blueprint('auth', __name__)

@auth.route('/signup', methods = ('GET', 'POST'))
def signup():
    if request.method == 'POST':
        email = request.form['email']
        username = request.form['username']
        password = generate_password_hash(request.form['password'])
        
        user = User(
            email = email,
            username = username,
            password = password
        )
            
        try:
            user.save()
            flash(f'Account successfully created.', 'success')
        except Exception as e:
            flash(f'Email or username already in use.', 'error')
            # return "Error \n %s" % (e)
        
        
    return render_template('signup.html')

class User(db.Document):
    email = db.EmailField(
        required = True
    )
    username = db.StringField(
        unique = True,
        required = True
    )
    password = db.StringField(
        required = True
    )
    
    meta = {
        "indexes": [
            {
                "fields": ["email"],
                "unique": True,
                "collation": { "locale": "en", "strength": 2 } }
            }
        ]
    }