feat(transactions): fix create mutation, add shouldUpdateBalance

[brewgator]

Summary

• Use merchantName field instead of merchant object in create mutation (matches Monarch's actual API schema)
• Make categoryId optional — only send when non-empty
• Add ShouldUpdateBalance param for manual account balance control
• Round amount to 2 decimal places before sending
• Parse fieldErrors from mutation response for better error messages
• Skip redundant getTransaction call after create — the previous implementation called s.Get() after a successful create, which was failing with "Something went wrong while processing: ['getTransaction']". Now returns the ID directly from the create response.
• Update create.graphql to match Monarch's actual mutation schema (Common_CreateTransactionMutation)

Split from #19 per review feedback.

Test plan

• Existing tests pass (go test ./...)
• Transaction creation works with merchantName
• Transaction creation works without categoryId
• ShouldUpdateBalance: false prevents manual account balance changes

[eshaffer321]

I tried doing

client, _ := monarch.NewClientWithToken(os.Getenv("MONARCH_TOKEN"))

shouldUpdateBalance := false
txn, err := client.Transactions.Create(context.Background(), &monarch.CreateTransactionParams{
    Date: monarch.Date{Time: time.Now().UTC()},
    AccountID: "a-real-account-id-from-Accounts.List",
    Amount: -0.01,
    Merchant: &monarch.Merchant{Name: "Codex PR21 Smoke Test"},
    Notes: "Temporary PR #21 smoke test",
    ShouldUpdateBalance: &shouldUpdateBalance,
})

fmt.Printf("txn=%#v err=%v\n", txn, err)

Result:

txn=<nil> err=failed to create transaction: error: BAD_REQUEST

With a response-body capture on the HTTP client, Monarch returned:

{"errors":[{"message":"Something went wrong while processing: None on request_id: None.","locations":[{"line":1,"column":43}]}]}

I also tried the raw GraphQL mutation directly with operationName included, to rule out this Go client wrapper:

curl https://api.monarch.com/graphql \
  -H "Authorization: Token $MONARCH_TOKEN" \
  -H "Content-Type: application/json" \
  -H "Accept: application/json" \
  -H "Client-Platform: web" \
  -H "Origin: https://app.monarchmoney.com" \
  --data '{
    "operationName":"Common_CreateTransactionMutation",
    "query":"mutation Common_CreateTransactionMutation($input: CreateTransactionMutationInput!) { createTransaction(input: $input) { transaction { id } errors { message code } } }",
    "variables":{
      "input":{
        "date":"2026-05-20",
        "accountId":"real-account-id",
        "amount":-0.01,
        "merchantName":"Codex PR21 Smoke Test",
        "notes":"Temporary smoke test",
        "shouldUpdateBalance":false
      }
    }
  }'

That produced the same HTTP 400 style error. I also tried updateBalance:false instead of shouldUpdateBalance:false. Do you have a working example I can try?

[brewgator]

The issue is that categoryId is required by the Monarch API. The Python client treats it as a mandatory positional argument — it's always sent in the input.

Your test and the curl example both omit categoryId, which causes the generic "Something went wrong while processing" error from Monarch's backend.

To fix: pass a valid categoryId. You can get one from client.Transactions.Categories.List() — the "Uncategorized" category works as a catch-all. For example:

categories, _ := client.Transactions.Categories.List(context.Background())
// Find "Uncategorized" or use any valid category ID

shouldUpdateBalance := false
txn, err := client.Transactions.Create(context.Background(), &monarch.CreateTransactionParams{
    Date:                monarch.Date{Time: time.Now().UTC()},
    AccountID:           "a-real-account-id",
    Amount:              -0.01,
    Merchant:            &monarch.Merchant{Name: "Codex PR21 Smoke Test"},
    CategoryID:          uncategorizedID, // required by Monarch
    Notes:               "Temporary PR #21 smoke test",
    ShouldUpdateBalance: &shouldUpdateBalance,
})

I'll update the code to always send categoryId (even when empty, defaulting to the value the caller provides) and add a doc comment noting it's required. Alternatively, we could make the Go client error early if CategoryID is empty. What's your preference?

[eshaffer321]

The issue is that categoryId is required by the Monarch API. The Python client treats it as a mandatory positional argument — it's always sent in the input.

Your test and the curl example both omit categoryId, which causes the generic "Something went wrong while processing" error from Monarch's backend.

To fix: pass a valid categoryId. You can get one from client.Transactions.Categories.List() — the "Uncategorized" category works as a catch-all. For example:

categories, _ := client.Transactions.Categories.List(context.Background())
// Find "Uncategorized" or use any valid category ID

shouldUpdateBalance := false
txn, err := client.Transactions.Create(context.Background(), &monarch.CreateTransactionParams{
    Date:                monarch.Date{Time: time.Now().UTC()},
    AccountID:           "a-real-account-id",
    Amount:              -0.01,
    Merchant:            &monarch.Merchant{Name: "Codex PR21 Smoke Test"},
    CategoryID:          uncategorizedID, // required by Monarch
    Notes:               "Temporary PR #21 smoke test",
    ShouldUpdateBalance: &shouldUpdateBalance,
})

I'll update the code to always send categoryId (even when empty, defaulting to the value the caller provides) and add a doc comment noting it's required. Alternatively, we could make the Go client error early if CategoryID is empty. What's your preference?

Let's make it required and error early if its empty

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 48.86%. Comparing base (ba1d9e7) to head (2e393a3).

❌ Your project check has failed because the head coverage (48.86%) is below the target coverage (70.00%). You can increase the head coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #21      +/-   ##
==========================================
+ Coverage   48.35%   48.86%   +0.51%     
==========================================
  Files          17       17              
  Lines        2031     2032       +1     
==========================================
+ Hits          982      993      +11     
+ Misses        959      953       -6     
+ Partials       90       86       -4     

Flag	Coverage Δ
unittests	48.86% <100.00%> (+0.51%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
pkg/monarch/transactions.go	76.05% <100.00%> (+2.55%)	⬆️

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