Don't scaffold default index method annotation

When scaffolding, we have an Npgsql-specific IndexMethod annotation to
represent PostgreSQL's index methods. Previously, we would always
output this annotation on indices, even when the method was the
default (btree); NpgsqlAnnotationCodeGenerator would elide it as by-
convention.

Although this is cleaner, it caused issues whenever two indices where
defined on the same column:
dotnet/efcore#11846

We now scaffold the annotation only for non-default index methods as a
workaround; the issue should now affect much less people.

Fixes #228

src/EFCore.PG/Scaffolding/Internal/NpgsqlDatabaseModelFactory.cs

@@ -513,7 +513,14 @@ NOT indisprimary
                             if (predicate != null)
                                 index.Filter = predicate;
 
-                            index[NpgsqlAnnotationNames.IndexMethod] = record.GetValueOrDefault<string>("amname");
+                            // It's cleaner to always output the index method on the database model,
+                            // even when it's btree (the default);
+                            // NpgsqlAnnotationCodeGenerator can then omit it as by-convention.
+                            // However, because of https://github.com/aspnet/EntityFrameworkCore/issues/11846 we omit
+                            // the annotation from the model entirely.
+                            var indexMethod = record.GetValueOrDefault<string>("amname");
+                            if (indexMethod != "btree")
+                                index[NpgsqlAnnotationNames.IndexMethod] = indexMethod;
 
                             table.Indexes.Add(index);
                         }

test/EFCore.PG.FunctionalTests/Scaffolding/NpgsqlDatabaseModelFactoryTest.cs

@@ -1505,8 +1505,14 @@ public void Index_method()
                     var methodIndex = table.Indexes.Single(i => i.Name == "ix_a");
                     Assert.Equal("hash", methodIndex.FindAnnotation(NpgsqlAnnotationNames.IndexMethod).Value);
 
+                    // It's cleaner to always output the index method on the database model,
+                    // even when it's btree (the default);
+                    // NpgsqlAnnotationCodeGenerator can then omit it as by-convention.
+                    // However, because of https://github.com/aspnet/EntityFrameworkCore/issues/11846 we omit
+                    // the annotation from the model entirely.
                     var noMethodIndex = table.Indexes.Single(i => i.Name == "ix_b");
-                    Assert.Equal("btree", noMethodIndex.FindAnnotation(NpgsqlAnnotationNames.IndexMethod).Value);
+                    Assert.Null(noMethodIndex.FindAnnotation(NpgsqlAnnotationNames.IndexMethod));
+                    //Assert.Equal("btree", noMethodIndex.FindAnnotation(NpgsqlAnnotationNames.IndexMethod).Value);
                 },
                 @"DROP TABLE ""IndexMethod""");
         }