Preserve collation when changing column type (#3479)

Fixes #3476

Author: roji

src/EFCore.PG/Migrations/NpgsqlMigrationsSqlGenerator.cs

@@ -456,9 +456,15 @@ protected override void Generate(AlterColumnOperation operation, IModel? model,
                 .Append("TYPE ")
                 .Append(type);
 
-            if (newCollation != oldCollation)
+            if (newCollation is not null)
             {
-                builder.Append(" COLLATE ").Append(DelimitIdentifier(newCollation ?? "default"));
+                builder.Append(" COLLATE ").Append(DelimitIdentifier(newCollation));
+            }
+            else if (type == oldType)
+            {
+                // If the type is the same, make it more explicit that we're just resetting the collation to the default
+                // (this isn't really required)
+                builder.Append(" COLLATE ").Append(DelimitIdentifier("default"));
             }
 
             builder.AppendLine(";");

test/EFCore.PG.FunctionalTests/Migrations/MigrationsNpgsqlTest.cs

@@ -969,6 +969,30 @@ public override async Task Alter_column_change_type()
         AssertSql("""ALTER TABLE "People" ALTER COLUMN "SomeColumn" TYPE bigint;""");
     }
 
+    [Fact]
+    public virtual async Task Alter_column_change_type_preserves_collation()
+    {
+        await Test(
+            builder => builder.Entity("People").Property<int>("Id"),
+            builder => builder.Entity("People").Property<string>("SomeColumn")
+                .HasColumnType("varchar")
+                .UseCollation(NonDefaultCollation),
+            builder => builder.Entity("People").Property<string>("SomeColumn")
+                .HasColumnType("text")
+                .UseCollation(NonDefaultCollation),
+            model =>
+            {
+                var table = Assert.Single(model.Tables);
+                var column = Assert.Single(table.Columns, c => c.Name == "SomeColumn");
+                Assert.Equal(NonDefaultCollation, column.Collation);
+            });
+
+        AssertSql(
+            """
+ALTER TABLE "People" ALTER COLUMN "SomeColumn" TYPE text COLLATE "POSIX";
+""");
+    }
+
     public override async Task Alter_column_make_required()
     {
         await base.Alter_column_make_required();