feature:implement ticket option - one ticket per user

[22f1001635]

This PR enforces a one-ticket-per-user limit when the one_ticket_per_user event setting is enabled. The validation:

• Checks the attendee's email against existing cart positions and paid/pending orders
• Shows clear error messages if the limit is exceeded
• Works alongside existing cart validation rules

No changes to existing behavior when the setting is disabled.

The restriction helps prevent duplicate ticket purchases while maintaining a smooth user experience.

Fixes #259

Summary by Sourcery

Implement a one-ticket-per-user restriction for event ticket purchases when enabled

New Features:

• Add a configurable setting to limit ticket purchases to one per user for an event

Bug Fixes:

• Resolve issue with potential multiple ticket purchases by the same user

Enhancements:

• Implement validation to prevent duplicate ticket purchases by checking against existing cart positions and orders

[sourcery-ai]

Reviewer's Guide by Sourcery

This PR implements a 'one ticket per user' feature. It adds a setting to the event configuration and enforces the restriction during the cart validation process by checking the attendee's email against existing cart positions and orders.

Sequence diagram for cart validation with one-ticket-per-user enabled

sequenceDiagram
    participant User
    participant CartService
    participant CartPosition
    participant OrderPosition
    participant Order

    User->>CartService: Adds item to cart
    CartService->>CartService: _check_item_constraints()
    alt event.settings.one_ticket_per_user is enabled
        CartService->>User: Get user email
        User->>CartService: Returns user email
        CartService->>CartPosition: Check existing cart positions with the same email
        CartPosition-->>CartService: Returns existing cart positions
        CartService->>OrderPosition: Check existing order positions with the same email
        OrderPosition-->>CartService: Returns existing order positions
        alt existing cart positions or orders found
            CartService-->>User: Raise CartError (one_ticket_per_user)
        else no existing cart positions or orders found
            CartService->>CartService: Continue with other validations
        end
    else event.settings.one_ticket_per_user is disabled
        CartService->>CartService: Continue with other validations
    end
    CartService-->>User: Item added to cart (or error)

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File-Level Changes

Change	Details	Files
Implements one-ticket-per-user restriction by validating the attendee's email against existing cart positions and paid/pending orders.	Added a new error message for the one-ticket-per-user restriction. Checks if the one_ticket_per_user setting is enabled. Retrieves the user's email address from the widget data. Queries existing cart positions and orders associated with the user's email. Raises a CartError if existing positions or orders are found. Added email address is required validation.	src/pretix/base/services/cart.py
Adds a setting to the event settings form to enable or disable the one-ticket-per-user restriction.	Added a one_ticket_per_user field to the TicketSettingsForm. Added a condition to remove the one_ticket_per_user field if attendee_emails_asked is not enabled. Added a help text to the one_ticket_per_user field. Added a translation for the one_ticket_per_user label and help text.	src/pretix/control/forms/event.py src/pretix/control/templates/pretixcontrol/event/tickets.html

Possibly linked issues

• Implement ticket option - one ticket per user #259: The PR implements the ticket option that limits tickets to one per user, as requested in the issue.

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[sourcery-ai]

Hey @22f1001635 - I've reviewed your changes - here's some feedback:

Overall Comments:

• Consider adding a database index on attendee_email in CartPosition and OrderPosition to improve query performance.
• It might be better to perform the email check in a separate service function to keep the cart service focused.

Here's what I looked at during the review

• 🟡 General issues: 1 issue found
• 🟢 Security: all looks good
• 🟢 Testing: all looks good
• 🟡 Complexity: 1 issue found
• 🟢 Documentation: all looks good

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[sourcery-ai]

issue (complexity): Consider extracting the 'one ticket per user' conditional into a helper method to improve readability and reduce redundancy by avoiding duplicate setting checks and reducing nesting complexity within the main function.

Consider extracting the "one ticket per user" conditional into its own helper method to reduce nesting and eliminate the redundant setting check. For example, you could add a helper similar to:

def _validate_one_ticket(self, error_messages):
    user_email = self._widget_data.get('email')
    if not user_email:
        raise CartError(_('Email address is required for ticket purchase'))
    user_email = user_email.lower().strip()
    existing_positions = CartPosition.objects.filter(
        event=self.event,
        attendee_email=user_email,
    ).exclude(
        pk__in=[op.position.id for op in self._operations if isinstance(op, self.RemoveOperation)]
    )
    existing_orders = OrderPosition.objects.filter(
        order__event=self.event,
        attendee_email=user_email,
        order__status__in=[Order.STATUS_PENDING, Order.STATUS_PAID]
    )
    if existing_positions.exists() or existing_orders.exists():
        raise CartError(error_messages['one_ticket_per_user'])

Then simplify your main flow by replacing the nested block with a single call:

if self.event.settings.one_ticket_per_user:
    self._validate_one_ticket(error_messages)

This flattening avoids duplicate setting checks and clarifies the one-ticket validation logic without altering functionality.

[norbusan]

@22f1001635 please consider this suggestion

[norbusan]

How have you tested these changes?
Do you have unit tests to be added?