accruals: AccrualPostings only reports inconsistent cost per currency.

Of course if an accrual has multiple currencies, it'll probably have
different costs, and that's fine.

conservancy_beancount/reports/accrual.py

@@ -144,7 +144,6 @@ class AccrualPostings(core.RelatedPostings):
     _FIELDS: Dict[str, Callable[[data.Posting], MetaValue]] = {
         'account': operator.attrgetter('account'),
         'contract': _meta_getter('contract'),
-        'cost': operator.attrgetter('cost'),
         'entity': _meta_getter('entity'),
         'invoice': _meta_getter('invoice'),
         'purchase_order': _meta_getter('purchase-order'),
@@ -157,8 +156,6 @@ class AccrualPostings(core.RelatedPostings):
         'accounts',
         'contract',
         'contracts',
-        'cost',
-        'costs',
         'entity',
         'entitys',
         'entities',
@@ -226,6 +223,17 @@ class AccrualPostings(core.RelatedPostings):
                         get_func(post),
                     )
                     yield Error(post.meta, errmsg, post.meta.txn)
+        costs = collections.defaultdict(set)
+        for post in self:
+            costs[post.units.currency].add(post.cost)
+        for code, currency_costs in costs.items():
+            if len(currency_costs) > 1:
+                for post in self:
+                    if post.units.currency == code:
+                        errmsg = 'inconsistent cost for invoice {}: {}'.format(
+                            self.invoice or "<none>", post.cost,
+                        )
+                        yield Error(post.meta, errmsg, post.meta.txn)
 
     def is_paid(self, default: Optional[bool]=None) -> Optional[bool]:
         if self.accrual_type is None:

tests/books/accruals.beancount

@@ -41,8 +41,8 @@
   rt-id: "rt:505"
   invoice: "rt:505/5050"
   approval: "rt:505/5040"
-  Income:Donations  -1000.00 USD
+  Income:Donations  -2,500 EUR {1.100 USD}
-  Assets:Receivable:Accounts  1000.00 USD
+  Assets:Receivable:Accounts  2,500 EUR {1.100 USD}
 
 2020-05-10 * "Lawyer" "April legal services"
   rt-id: "rt:510"
@@ -60,8 +60,8 @@
 2020-05-20 * "DonorA" "Donation made"
   rt-id: "rt:505"
   invoice: "rt:505/5050"
-  Assets:Receivable:Accounts  -1000.00 USD
+  Assets:Receivable:Accounts  -2,750.00 USD
-  Assets:Checking  1000.00 USD
+  Assets:Checking  2,750.00 USD
   receipt: "DonorAWire.pdf"
 
 2020-05-25 * "Lawyer" "May payment"

tests/test_reports_accrual.py

@@ -212,20 +212,6 @@ def test_accrual_postings_consistent_account(acct_name):
     assert related.account == acct_name
     assert related.accounts == {acct_name}
 
-@pytest.mark.parametrize('cost', [
-    testutil.Cost('1.2', 'USD'),
-    None,
-])
-def test_accrual_postings_consistent_cost(cost):
-    meta = {'invoice': 'FXinvoice.pdf'}
-    txn = testutil.Transaction(postings=[
-        (ACCOUNTS[0], 60, 'EUR', cost, meta),
-        (ACCOUNTS[0], 30, 'EUR', cost, meta),
-    ])
-    related = accrual.AccrualPostings(data.Posting.from_txn(txn))
-    assert related.cost == cost
-    assert related.costs == {cost}
-
 @pytest.mark.parametrize('meta_key,acct_name', testutil.combine_values(
     CONSISTENT_METADATA,
     ACCOUNTS,
@@ -255,20 +241,6 @@ def test_accrual_postings_inconsistent_account():
     assert related.account is related.INCONSISTENT
     assert related.accounts == set(ACCOUNTS)
 
-def test_accrual_postings_inconsistent_cost():
-    meta = {'invoice': 'FXinvoice.pdf'}
-    costs = {
-        testutil.Cost('1.1', 'USD'),
-        testutil.Cost('1.2', 'USD'),
-    }
-    txn = testutil.Transaction(postings=[
-        (ACCOUNTS[0], 10, 'EUR', cost, meta)
-        for cost in costs
-    ])
-    related = accrual.AccrualPostings(data.Posting.from_txn(txn))
-    assert related.cost is related.INCONSISTENT
-    assert related.costs == costs
-
 @pytest.mark.parametrize('meta_key,acct_name', testutil.combine_values(
     CONSISTENT_METADATA,
     ACCOUNTS,