Calculated Fields in Form Submission

Anvil Q&A
1

I have a Data Table called “items” with columns “price”, “deal_price”, and “percent_discount”, based on the CRUD app tutorial “Build a News Aggregator”

I only want the user to have to fill out the “price” and “deal_price”, and the “percent_discount” field in the Data Table to be calculated automatically when the row is created or updated.

What would be the best way/place to do this?

2

Assuming I have understood you correctly …

…you would calculate the percent_discount before you add or update the record. So for example -

pd = float( 100 - ((deal_price / price) * 100) )
row = app_tables.mytable.add(price=price, deal_price =deal_price, percent_discount=pd)

Is that what you mean?

1 Like
3

Sorry I’m new to to Python, so your answer makes sense to me but how do I apply it in the context of how the News Aggregator is set up with a dictionary?

Here is the server code:

@anvil.server.callable
def add_meal(meal_dict):
  app_tables.meals.add_row(
    created=datetime.now(),
    owner=anvil.users.get_user(),
    active=True,
    **meal_dict
  )

And here is where it is called from the form:

    def primary_color_1_click(self, **event_args):
    """This method is called when the button is clicked"""
    # Add meal button click
    new_meal ={}
    
    if alert(content=mealEdit(item=new_meal),title="Add Meal",
             large=True, buttons=[("Save", True),("Cancel", False)]):
       anvil.server.call('add_meal', new_meal)
4

I’m not familiar with the application, but if the dictionary new_meal contains the data to be added to the table, then I’m assuming it looks something like this (using your original question’s parameters and using made up values for illustration) :

{“deal_price”:5.99, “price”:9.99}

This assumes “percent_discount” has not yet been added as it has not yet been calculated.

So your server function might now look something like this :

@anvil.server.callable
def add_meal(meal_dict):

  # Print some debug to confirm meal_dict contains what you think it does.
  print("meal_dict = ", meal_dict) 

  # Assign these vars to make the calculation line a bit more readable.
  deal_price = float(meal_dict['deal_price'])
  price = float(meal_dict['price'])

  # Insert a new key-value into the dictionary
  meal_dict['percent_discount'] = float( 100 - ((deal_price / price) * 100) )

  app_tables.meals.add_row(
    created=datetime.now(),
    owner=anvil.users.get_user(),
    active=True,
    **meal_dict
  )

Does that make sense (with the comments)?

2 Likes
5

Awesome thank you so much! This worked perfectly.

1 Like