lidavidm commented on a change in pull request #11704:
URL: https://github.com/apache/arrow/pull/11704#discussion_r750278502



##########
File path: cpp/src/arrow/dataset/test_util.h
##########
@@ -631,12 +646,92 @@ class FileFormatScanMixin : public 
FileFormatFixtureMixin<FormatHelper>,
     for (auto maybe_batch : PhysicalBatches(fragment)) {
       ASSERT_OK_AND_ASSIGN(auto batch, maybe_batch);
       row_count += batch->num_rows();
-      AssertSchemaEqual(*batch->schema(), *expected_schema,
-                        /*check_metadata=*/false);
+      ASSERT_THAT(
+          batch->schema()->fields(),
+          ::testing::UnorderedPointwise(PointeesEquals(), 
expected_schema->fields()))
+          << "EXPECTED:\n"
+          << expected_schema->ToString() << "\nACTUAL:\n"
+          << batch->schema()->ToString();
     }
 
     ASSERT_EQ(row_count, expected_rows());
   }
+  void TestScanProjectedNested(bool fine_grained_selection = false) {
+    auto f32 = field("f32", float32());
+    auto f64 = field("f64", float64());
+    auto i32 = field("i32", int32());
+    auto i64 = field("i64", int64());
+    auto struct1 = field("struct1", struct_({f32, i32}));
+    auto struct2 = field("struct2", struct_({f64, i64, struct1}));
+    this->SetSchema({struct1, struct2, f32, f64, i32, i64});
+    this->ProjectNested({".struct1.f32", ".struct2.struct1", 
".struct2.struct1.f32"});
+    this->SetFilter(equal(field_ref(FieldRef("struct2", "i64")), literal(0)));
+
+    std::shared_ptr<Schema> expected_schema;
+    if (fine_grained_selection) {
+      // Some formats, like Parquet, let you pluck only a part of a complex 
type
+      expected_schema = schema({
+          field("struct1", struct_({f32})),
+          field("struct2", struct_({i64, struct1})),
+      });
+    } else {
+      expected_schema = schema({struct1, struct2});
+    }
+
+    {
+      auto reader = this->GetRecordBatchReader(opts_->dataset_schema);
+      auto source = this->GetFileSource(reader.get());
+      auto fragment = this->MakeFragment(*source);
+
+      int64_t row_count = 0;
+      for (auto maybe_batch : PhysicalBatches(fragment)) {
+        ASSERT_OK_AND_ASSIGN(auto batch, maybe_batch);
+        row_count += batch->num_rows();
+        ASSERT_THAT(
+            batch->schema()->fields(),
+            ::testing::UnorderedPointwise(PointeesEquals(), 
expected_schema->fields()))
+            << "EXPECTED:\n"
+            << expected_schema->ToString() << "\nACTUAL:\n"
+            << batch->schema()->ToString();
+      }
+      ASSERT_EQ(row_count, expected_rows());
+    }
+    {
+      // File includes an extra child in struct2

Review comment:
       We can handle missing fields just fine because (once ARROW-1888 is 
implemented) we can synthesize a null child to stand in for it. But, we can't 
handle a duplicate name because it's ambiguous which child we're referring to.




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