Fix tags getting removed after / during bank sync (#634)

* fix: Preserve transaction tags during rule application

When rules set tags, they now ADD to existing tags instead of replacing
them. This fixes issue #518 where tags were being removed during bank sync.

The root cause was that SetTransactionTags called enrich_attribute with
just the single tag from the rule, which replaced all existing tags.
Now it merges the new tag with existing tags using .uniq to prevent
duplicates.

This preserves:
- User-applied tags that shouldn't be overwritten by rules
- Tags from other rules when multiple rules match the same transaction
- Tags set during previous syncs

* fix: Add nil guard for tag in SetTransactionTags

Return early with 0 if the tag is not found, preventing NoMethodError
when find_by_id returns nil. This matches the pattern used in
SetTransactionMerchant.

---------

Co-authored-by: Claude <noreply@anthropic.com>

app/models/rule/action_executor/set_transaction_tags.rb

@@ -9,6 +9,7 @@ class Rule::ActionExecutor::SetTransactionTags < Rule::ActionExecutor
 
   def execute(transaction_scope, value: nil, ignore_attribute_locks: false, rule_run: nil)
     tag = family.tags.find_by_id(value)
+    return 0 unless tag
 
     scope = transaction_scope
 
@@ -17,9 +18,14 @@ class Rule::ActionExecutor::SetTransactionTags < Rule::ActionExecutor
     end
 
     count_modified_resources(scope) do |txn|
+      # Merge the new tag with existing tags instead of replacing them
+      # This preserves tags set by users or other rules
+      existing_tag_ids = txn.tag_ids || []
+      merged_tag_ids = (existing_tag_ids + [ tag.id ]).uniq
+
       txn.enrich_attribute(
         :tag_ids,
-        [ tag.id ],
+        merged_tag_ids,
         source: "rule"
       )
     end

test/models/account/provider_import_adapter_test.rb

@@ -1219,6 +1219,41 @@ class Account::ProviderImportAdapterTest < ActiveSupport::TestCase
     end
   end
 
+  test "preserves transaction tags when re-importing existing transaction" do
+    tag = Tag.create!(name: "Salary", family: @family)
+
+    # Create initial transaction
+    entry = @adapter.import_transaction(
+      external_id: "plaid_tag_test",
+      amount: 1000.00,
+      currency: "USD",
+      date: Date.today,
+      name: "Paycheck",
+      source: "plaid"
+    )
+
+    # Add tag to the transaction (simulating user or rule action)
+    entry.transaction.tags << tag
+    entry.transaction.save!
+    assert_equal [ tag ], entry.transaction.reload.tags
+
+    # Re-import the same transaction with updated data
+    assert_no_difference "@account.entries.count" do
+      updated_entry = @adapter.import_transaction(
+        external_id: "plaid_tag_test",
+        amount: 1000.00,
+        currency: "USD",
+        date: Date.today,
+        name: "Updated Paycheck Name",
+        source: "plaid"
+      )
+
+      assert_equal entry.id, updated_entry.id
+      # Tags should be preserved
+      assert_equal [ tag ], updated_entry.transaction.reload.tags
+    end
+  end
+
   test "Plaid pending_transaction_id takes priority over amount matching" do
     # Create TWO pending transactions with same amount
     pending1 = @adapter.import_transaction(

test/models/rule/action_test.rb

@@ -59,6 +59,51 @@ class Rule::ActionTest < ActiveSupport::TestCase
     end
   end
 
+  test "set_transaction_tags preserves existing tags" do
+    existing_tag = @family.tags.create!(name: "Existing tag")
+    new_tag = @family.tags.create!(name: "New tag from rule")
+
+    # Add existing tag to transaction
+    @txn2.tags << existing_tag
+    @txn2.save!
+    assert_equal [ existing_tag ], @txn2.reload.tags
+
+    action = Rule::Action.new(
+      rule: @transaction_rule,
+      action_type: "set_transaction_tags",
+      value: new_tag.id
+    )
+
+    action.apply(@rule_scope)
+
+    # Transaction should have BOTH the existing tag and the new tag
+    @txn2.reload
+    assert_includes @txn2.tags, existing_tag
+    assert_includes @txn2.tags, new_tag
+    assert_equal 2, @txn2.tags.count
+  end
+
+  test "set_transaction_tags does not duplicate existing tags" do
+    tag = @family.tags.create!(name: "Single tag")
+
+    # Add tag to transaction
+    @txn2.tags << tag
+    @txn2.save!
+    assert_equal [ tag ], @txn2.reload.tags
+
+    action = Rule::Action.new(
+      rule: @transaction_rule,
+      action_type: "set_transaction_tags",
+      value: tag.id
+    )
+
+    action.apply(@rule_scope)
+
+    # Transaction should still have just one tag (not duplicated)
+    @txn2.reload
+    assert_equal [ tag ], @txn2.tags
+  end
+
   test "set_transaction_merchant" do
     merchant = @family.merchants.create!(name: "Rule test merchant")