input: Write a python function to transform and migrate data from a MySQL database to a MongoDB collection.
  assert transform_and_migrate_data() == "Migration successful"
  assert len(list(db.customers.find({}))) > 0
output: def transform_and_migrate_data():
      import pymysql
      from pymongo import MongoClient
  
      # Connect to MySQL database
      mysql_conn = pymysql.connect(
          host='localhost',
          user='your_mysql_username',
          password='your_mysql_password',
          database='your_mysql_database'
      )
  
      # Connect to MongoDB
      mongo_client = MongoClient('localhost', 27017)
      mongo_db = mongo_client['your_mongodb_database']
      mongo_collection = mongo_db['customers']
  
      try:
          with mysql_conn.cursor() as cursor:
              # Fetch data from MySQL
              cursor.execute("SELECT id, name, email FROM customers")
              rows = cursor.fetchall()
  
              # Transform and insert data into MongoDB
              for row in rows:
                  document = {
                      'customer_id': row[0],
                      'name': row[1],
                      'email': row[2]
                  }
                  mongo_collection.insert_one(document)
      finally:
          mysql_conn.close()
          mongo_client.close()
  
      return "Migration successful"