fix: multi-tax detection and net_amount calculation

[0xD0M1M0]

PR Description

This PR addresses two connected issues when generating e-invoices for Sales Invoices with multiple tax rows or configurations where a row in Sales Taxes and Charges is present but not applicable.

Example: Sales Invoice ACC-SINV-2025-00035

#	Item	Net Amount	Item Tax Template
1	PM0003	105.00	19% - CP
2	PM0001	103.00	7% - CP
3	PM0001	114.00	19% - CP

#	Account	Rate (row)	Tax Amount
1	3806 - Umsatzsteuer 19%	0	41.61
2	3801 - Umsatzsteuer 7%	0	7.21

Expected e-invoice settlement lines:

• 19% basis = 105 + 114 = 219.00, tax = 41.61
• 7% basis = 103 = 103.00, tax = 7.21

1) Basis amount for multi-tax "On Net Total" rows

In multi-tax environments, ERPNext v15 does not provide a net_amount on tax rows for use as the taxable basis. The existing logic relied on fallback options (hasattr(tax, "net_amount")) that could match an uninitialized or zero-valued field, resulting in basis_amount = 0.

Without this PR:

• If hasattr(tax, "net_amount") evaluated to True (for example due to a Custom Field or attribute initialization on the site), the basis could become 0 or any other value for both tax lines, producing an incorrect e-invoice tax basis.

With this PR:

• The basis is first derived from tax_amount / rate * 100 (here: 41.61 / 19 * 100 = 219.0 and 7.21 / 7 * 100 = 103.0).
• If the derived value is exact at 2 decimals, it is used directly.
• If not exact (rounding would be required), it falls back to summing line-item net_amount values whose resolved VAT rate matches the tax row's rate.

This keeps the fast path for exact results and guarantees a correct basis when rounding would otherwise distort the taxable amount.

2) Highest non-zero rate from Item Tax Template

When multiple accounts are listed in Sales Taxes and Charges and an Item Tax Template contains a matching account with 0% tax, the old logic returned the first match, which could be 0%.

Without this PR:

• A template could contain:
  • Row A: tax_type = "3806 - Umsatzsteuer 19%", tax_rate = 0
  • Row B: tax_type = "3801 - Umsatzsteuer 7%", tax_rate = 7
• Because both accounts are present in the invoice tax table, the old first-match behavior could return 0% for a line that should be 7%.

With this PR:

• Matching entries with tax_rate = 0 are excluded in the applicable-account matching path.
• The highest non-zero matching tax_rate is returned.
• If income_account is available, direct account matching is still preferred.

This avoids false 0% assignments and keeps mixed-tax invoices consistent.

[greptile-apps]

Greptile Summary

This PR fixes two bugs in e-invoice generation for Sales Invoices with multiple tax rows: incorrect basis-amount calculation for multi-rate "On Net Total" rows, and false 0% VAT rate assignment when an Item Tax Template contains a zero-rate entry for a matched account.

• Basis amount: replaces the unreliable hasattr(tax, "net_amount") fast path with _sum_item_net_matching_on_net_total_rate, which sums net_amount of items whose resolved VAT rate matches the current tax row, falling back to the calculated tax_amount / rate * 100 approach.
• Rate resolution: get_item_rate now skips zero-rate template rows on ERPNext v15 and rows flagged not_applicable on v16+, preventing false 0% assignments; also upgrades from frappe.get_doc to frappe.get_cached_doc to reduce per-item DB hits.
• Row skipping: adds an early continue for On Net Total rows where tax_amount == 0 and tax_rate != 0, avoiding validation errors for inapplicable tax rows.

Confidence Score: 4/5

Safe to merge for the common case the PR targets, but invoices that mix items with and without Item Tax Templates under multiple tax rates can get a wrong BasisAmount without any error signal

The _sum_item_net_matching_on_net_total_rate method silently returns a partial sum when some items lack item_tax_template in a multi-tax invoice, and because the fallback chain is or-chained, a non-zero partial sum prevents the reliable calculated fallback from firing. The emitted BasisAmount would be lower than the actual taxable amount for that rate, potentially causing downstream schema validation failures.

eu_einvoice/european_e_invoice/custom/sales_invoice.py — specifically the or-chain in _add_taxes_and_charges (lines 574–581) and the _sum_item_net_matching_on_net_total_rate method that feeds it

Important Files Changed

Filename	Overview
eu_einvoice/european_e_invoice/custom/sales_invoice.py	Refactors multi-tax basis-amount calculation and VAT rate resolution; introduces _sum_item_net_matching_on_net_total_rate which can emit a partial sum when items lack item_tax_template, silently suppressing the calculated fallback

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    A[_add_taxes_and_charges loop] --> B{charge_type == On Net Total?}
    B -- No --> Z[Other charge types]
    B -- Yes --> C{tax_amount == 0 AND tax_rate != 0?}
    C -- Yes --> SKIP[continue / skip row]
    C -- No --> D[len taxes == 1?]
    D -- Yes --> E[basis = invoice.net_total]
    D -- No --> F[_sum_item_net_matching_on_net_total_rate]
    F --> G{sum > 0?}
    G -- Yes --> H[basis = item net sum]
    G -- No --> I{hasattr net_amount?}
    I -- Yes --> J[basis = tax.net_amount]
    I -- No --> K{hasattr custom_net_amount?}
    K -- Yes --> L[basis = tax.custom_net_amount]
    K -- No --> M{tax_amount and rate?}
    M -- Yes --> N[basis = tax_amount / rate * 100]
    M -- No --> O[basis = 0]

    subgraph get_item_rate
        P[item_tax_template provided?] -- Yes --> Q[frappe.get_cached_doc template]
        Q --> R{v16 not_applicable field?}
        R -- Yes --> S[match: tax_type in accounts AND NOT not_applicable]
        R -- No --> T[match: tax_type in accounts AND tax_rate != 0]
        S --> U[return first matching rate]
        T --> U
        P -- No --> V{exactly 1 On Net Total row?}
        V -- Yes --> W[return that row rate]
        V -- No --> X[return None]
    end

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[barredterra]

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