fix(dbless): fix error data loss caused by weakly typed of function in declarative_config_flattened function

changelog/unreleased/kong/fix-declarative-config-flattened-data-loss.yml

@@ -0,0 +1,3 @@
+message: fix error data loss caused by weakly typed of function in declarative_config_flattened function
+type: bugfix
+scope: Configuration

kong/db/errors.lua

@@ -1033,7 +1033,13 @@ do
       for i, err_t_i in drain(section_errors) do
         local entity = entities[i]
 
-        if type(entity) == "table" then
+
+        -- promote error strings to `@entity` type errors
+        if type(err_t_i) == "string" then
+          err_t_i = { ["@entity"] = err_t_i }
+        end
+
+        if type(entity) == "table" and type(err_t_i) == "table" then
           add_entity_errors(entity_type, entity, err_t_i, flattened)
 
         else

spec/02-integration/04-admin_api/15-off_spec.lua

@@ -2697,6 +2697,28 @@ R6InCcH2Wh8wSeY5AuDXvu2tv9g/PW9wIJmPuKSHMA==
       },
     }, flattened)
   end)
+  it("origin error do not loss when enable flatten_errors - (#12167)", function()
+    local input = {
+      _format_version = "3.0",
+      consumers = {
+        {
+          id = "a73dc9a7-93df-584d-97c0-7f41a1bbce3d",
+          username = "test-consumer-1",
+        },
+        {
+          id = "a73dc9a7-93df-584d-97c0-7f41a1bbce32",
+          username = "test-consumer-1",
+        },
+      },
+    }
+
+    local flattened = post_config(input)
+
+    assert.equals(1, #flattened,
+                  "unexpected number of flattened errors")
+    assert.equals("uniqueness violation: 'consumers' entity with username set to 'test-consumer-1' already declared", flattened[1].errors[1].message)
+    assert.equals("entity", flattened[1].errors[1].type)
+  end)
 end)