refactor: totals mixins v2

[turip]

Overview

This patch moves the totals mixin to a seperate package while allowing adapters to reuse the set/map functions.

The motivation is twofold:

• Adding credits totals was a mess due to a lot of places containing totals mixin references.
• charges will require realizations as mixins as they will be used at different locations in the database. This approach gives us the flexibility required to maintain a consistent schema and logic on the code level.

Notes for reviewer

Summary by CodeRabbit

Release Notes

• Refactor
  • Consolidated billing totals handling by introducing a centralized totals package with helper functions and utilities, replacing scattered per-field assignments throughout invoice creation and updates.
  • Improved code maintainability through unified totals mapping, calculation, and mutation patterns using new helpers and Ent mixin accessors.

[coderabbitai]

📝 Walkthrough

Walkthrough

This PR refactors the Totals type out of the billing package into a dedicated openmeter/billing/models/totals package and introduces mixin helpers (Set, FromDB, Sum) for cleaner totals handling. It updates type references across 20+ files and adds an Ent framework extension for generating mixin accessor methods.

Changes

Cohort / File(s)	Summary
Totals Package Extraction openmeter/billing/models/totals/model.go, openmeter/billing/models/totals/mixin.go	New totals package with Totals struct, Mutator/TotalsGetter interfaces, and helper functions (Set, FromDB, Sum) for fluent totals manipulation via Ent mixins.
Ent Framework Extension pkg/framework/entutils/entmixinaccessor/entmixinaccessor.go, pkg/framework/entutils/entmixinaccessor/mixinaccessor.tpl	New Ent extension generating accessor methods for mixin-added fields; registered in openmeter/ent/entc.go.
Invoice Adapter Refactoring openmeter/billing/adapter/invoice.go, openmeter/billing/adapter/stdinvoicelinemapper.go, openmeter/billing/adapter/stdinvoicelines.go	Replace inline Totals field assignments with totals.Set and totals.FromDB helpers during invoice/line creation and updates.
Charge Service Updates openmeter/billing/charges/adapter/mapper.go, openmeter/billing/charges/adapter/stdinvoice.go, openmeter/billing/charges/chargeflatfee.go, openmeter/billing/charges/stdinvoice.go	Update Totals type references from billing.Totals to totals.Totals and apply totals.Set helper in accrued usage creation.
Type Reference Updates openmeter/billing/stdinvoice.go, openmeter/billing/stdinvoiceline.go, openmeter/billing/invoicedetailedline.go, openmeter/billing/derived.gen.go	Update public struct fields and function signatures to use totals.Totals instead of billing.Totals; regenerate derived equality helpers.
Service Layer Updates openmeter/billing/service/invoicecalc/details.go, openmeter/billing/service/lineservice/detailedlines.go, openmeter/billing/service/lineservice/linebase.go, openmeter/billing/service/lineservice/service.go	Replace inline totals accumulation with totals.Sum helper; update function return types to totals.Totals.
Ent Schema Consolidation openmeter/ent/schema/billing.go, openmeter/ent/schema/charges.go	Remove local TotalsMixin declaration and update all schemas to use external totals.Mixin.
Test Updates openmeter/billing/service/lineservice/usagebasedline_test.go, test/billing/invoice_test.go	Update test helpers and fixtures to reference totals.Totals type.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~25 minutes

Possibly related PRs

• refactor: split detailed line type #3514: Modifies DetailedLineBase.Totals type handling in the same invoice line mapping code
• refactor: reimplement min max commitments #2627: Refactors invoice mapping logic in the same adapter/invoice.go file
• chore: use derive for eq checks #3521: Updates derived equality helpers in derived.gen.go that this PR regenerates

Suggested labels

release-note/misc, kind/refactor, area/billing

Suggested reviewers

• chrisgacsal
• tothandras
• GAlexIHU

🚥 Pre-merge checks | ✅ 2 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 18.75% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (2 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title accurately captures the main change: refactoring the totals mixin to a separate package and exposing helper functions for adapter reuse.

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✨ Finishing Touches

• 📝 Generate docstrings (stacked PR)
• 📝 Generate docstrings (commit on current branch)

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch refactor/totals-mixin-v2

📝 Coding Plan

• Generate coding plan for human review comments

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[coderabbitai]

🧹 Nitpick comments (1)

openmeter/billing/service/lineservice/detailedlines.go (1)

202-231: Variable shadows package name - consider renaming for clarity.

The local variable totals on line 210 shadows the imported totals package. While this works correctly (all subsequent uses like totals.CalculateTotal() refer to the local struct's method, not a package function), it can be confusing to readers.

🔧 Suggested rename for clarity

-func calculateDetailedLineTotals(line billing.DetailedLine) (totals.Totals, error) {
+func calculateDetailedLineTotals(line billing.DetailedLine) (totals.Totals, error) {
 	// Calculate the line totals
 	calc, err := line.Currency.Calculator()
 	if err != nil {
 		return totals.Totals{}, err
 	}

 	// Calculate the line totals
-	totals := totals.Totals{
+	result := totals.Totals{
 		DiscountsTotal: line.AmountDiscounts.SumAmount(calc),
 		CreditsTotal:   line.CreditsApplied.SumAmount(calc),

 		// TODO[OM-979]: implement taxes
 		TaxesInclusiveTotal: alpacadecimal.Zero,
 		TaxesExclusiveTotal: alpacadecimal.Zero,
 		TaxesTotal:          alpacadecimal.Zero,
 	}

 	amount := calc.RoundToPrecision(line.PerUnitAmount.Mul(line.Quantity))

 	switch line.Category {
 	case billing.FlatFeeCategoryCommitment:
-		totals.ChargesTotal = amount
+		result.ChargesTotal = amount
 	default:
-		totals.Amount = amount
+		result.Amount = amount
 	}

-	totals.Total = totals.CalculateTotal()
+	result.Total = result.CalculateTotal()

-	return totals, nil
+	return result, nil
 }

🤖 Prompt for AI Agents

Verify each finding against the current code and only fix it if needed.

In `@openmeter/billing/service/lineservice/detailedlines.go` around lines 202 -
231, In calculateDetailedLineTotals, the local variable named totals shadows the
imported totals package; rename the local variable (e.g., lineTotals or
totalsStruct) and update all references inside calculateDetailedLineTotals
(fields like DiscountsTotal, CreditsTotal, ChargesTotal/Amount/Total and the
call to CalculateTotal()) to use the new name so there is no confusion between
the package and the local struct.

🤖 Prompt for all review comments with AI agents

Verify each finding against the current code and only fix it if needed.

Nitpick comments:
In `@openmeter/billing/service/lineservice/detailedlines.go`:
- Around line 202-231: In calculateDetailedLineTotals, the local variable named
totals shadows the imported totals package; rename the local variable (e.g.,
lineTotals or totalsStruct) and update all references inside
calculateDetailedLineTotals (fields like DiscountsTotal, CreditsTotal,
ChargesTotal/Amount/Total and the call to CalculateTotal()) to use the new name
so there is no confusion between the package and the local struct.

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro

Run ID: b660ba5f-1451-44b1-858c-397df6217745

📥 Commits

Reviewing files that changed from the base of the PR and between 699c828 and e973957.

⛔ Files ignored due to path filters (1)

• openmeter/ent/db/entmixinaccessor.go is excluded by !**/ent/db/**

📒 Files selected for processing (25)

• openmeter/billing/adapter/invoice.go
• openmeter/billing/adapter/stdinvoicelinemapper.go
• openmeter/billing/adapter/stdinvoicelines.go
• openmeter/billing/charges/adapter/mapper.go
• openmeter/billing/charges/adapter/stdinvoice.go
• openmeter/billing/charges/chargeflatfee.go
• openmeter/billing/charges/stdinvoice.go
• openmeter/billing/derived.gen.go
• openmeter/billing/httpdriver/invoice.go
• openmeter/billing/invoicedetailedline.go
• openmeter/billing/models/totals/mixin.go
• openmeter/billing/models/totals/model.go
• openmeter/billing/service/invoicecalc/details.go
• openmeter/billing/service/lineservice/detailedlines.go
• openmeter/billing/service/lineservice/linebase.go
• openmeter/billing/service/lineservice/service.go
• openmeter/billing/service/lineservice/usagebasedline_test.go
• openmeter/billing/stdinvoice.go
• openmeter/billing/stdinvoiceline.go
• openmeter/ent/entc.go
• openmeter/ent/schema/billing.go
• openmeter/ent/schema/charges.go
• pkg/framework/entutils/entmixinaccessor/entmixinaccessor.go
• pkg/framework/entutils/entmixinaccessor/mixinaccessor.tpl
• test/billing/invoice_test.go