From fa0bf7ef2cdd675dec6e46cef01a90258c566888 Mon Sep 17 00:00:00 2001
From: Shay Rojansky <roji@roji.org>
Date: Fri, 28 Feb 2025 17:24:00 +0200
Subject: [PATCH] Preserve collation when changing column type

Fixes #3476
---
 .../NpgsqlMigrationsSqlGenerator.cs           | 10 ++++++++--
 .../Migrations/MigrationsNpgsqlTest.cs        | 20 +++++++++++++++++--
 2 files changed, 26 insertions(+), 4 deletions(-)

diff --git a/src/EFCore.PG/Migrations/NpgsqlMigrationsSqlGenerator.cs b/src/EFCore.PG/Migrations/NpgsqlMigrationsSqlGenerator.cs
index 6e09ed6be..fccab7d18 100644
--- a/src/EFCore.PG/Migrations/NpgsqlMigrationsSqlGenerator.cs
+++ b/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(";");
diff --git a/test/EFCore.PG.FunctionalTests/Migrations/MigrationsNpgsqlTest.cs b/test/EFCore.PG.FunctionalTests/Migrations/MigrationsNpgsqlTest.cs
index 9d813d56f..c4ebc7bc3 100644
--- a/test/EFCore.PG.FunctionalTests/Migrations/MigrationsNpgsqlTest.cs
+++ b/test/EFCore.PG.FunctionalTests/Migrations/MigrationsNpgsqlTest.cs
@@ -964,9 +964,25 @@ await Test(
 
     public override async Task Alter_column_change_type()
     {
-        await base.Alter_column_change_type();
+        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 bigint;""");
+        AssertSql(
+            """
+ALTER TABLE "People" ALTER COLUMN "SomeColumn" TYPE text COLLATE "POSIX";
+""");
     }
 
     public override async Task Alter_column_make_required()