Migrations: Fix some bugs

Changes:
- Synthesize DEFAULT value when column altered to NOT NULL (fixes dotnet#19882)
- SQL Server: Rebuild indexes when column collation is altered (fixes dotnet#21547)

src/EFCore.Relational/Migrations/Internal/MigrationsModelDiffer.cs

@@ -1011,7 +1011,7 @@ protected virtual IEnumerable<MigrationOperation> Diff(
 
                 Initialize(
                     alterColumnOperation, target, targetTypeMapping,
-                    target.IsNullable, targetMigrationsAnnotations, inline: true);
+                    target.IsNullable, targetMigrationsAnnotations, inline: !source.IsNullable);
 
                 Initialize(
                     alterColumnOperation.OldColumn, source, sourceTypeMapping,

src/EFCore.SqlServer/Migrations/SqlServerMigrationsSqlGenerator.cs

@@ -283,7 +283,9 @@ protected override void Generate(
                         operation.Name,
                         operation.OldColumn,
                         model);
-                narrowed = type != oldType || !operation.IsNullable && operation.OldColumn.IsNullable;
+                narrowed = type != oldType
+                    || operation.Collation != operation.OldColumn.Collation
+                    || !operation.IsNullable && operation.OldColumn.IsNullable;
             }
 
             if (narrowed)

test/EFCore.Relational.Tests/Migrations/Internal/MigrationsModelDifferTest.cs

@@ -1912,6 +1912,48 @@ public void Alter_column_nullability()
                 });
         }
 
+        [ConditionalFact]
+        public void Alter_column_nullability_to_required()
+        {
+            Execute(
+                source => source.Entity(
+                    "Bison",
+                    x =>
+                    {
+                        x.ToTable("Bison", "dbo");
+                        x.Property<int>("Id");
+                        x.Property<string>("Name")
+                            .HasColumnType("nvarchar(30)")
+                            .IsRequired(false)
+                            .HasDefaultValueSql("CreateBisonName()");
+                    }),
+                target => target.Entity(
+                    "Bison",
+                    x =>
+                    {
+                        x.ToTable("Bison", "dbo");
+                        x.Property<int>("Id");
+                        x.Property<string>("Name")
+                            .HasColumnType("nvarchar(30)")
+                            .IsRequired()
+                            .HasDefaultValueSql("CreateBisonName()");
+                    }),
+                operations =>
+                {
+                    Assert.Equal(1, operations.Count);
+
+                    var operation = Assert.IsType<AlterColumnOperation>(operations[0]);
+                    Assert.Equal("dbo", operation.Schema);
+                    Assert.Equal("Bison", operation.Table);
+                    Assert.Equal("Name", operation.Name);
+                    Assert.Equal(typeof(string), operation.ClrType);
+                    Assert.Equal("nvarchar(30)", operation.ColumnType);
+                    Assert.False(operation.IsNullable);
+                    Assert.Equal(string.Empty, operation.DefaultValue);
+                    Assert.Equal("CreateBisonName()", operation.DefaultValueSql);
+                });
+        }
+
         [ConditionalFact]
         public void Alter_column_type()
         {

test/EFCore.SqlServer.FunctionalTests/MigrationsSqlServerTest.cs

@@ -502,7 +502,8 @@ FROM [sys].[default_constraints] [d]
 INNER JOIN [sys].[columns] [c] ON [d].[parent_column_id] = [c].[column_id] AND [d].[parent_object_id] = [c].[object_id]
 WHERE ([d].[parent_object_id] = OBJECT_ID(N'[People]') AND [c].[name] = N'SomeColumn');
 IF @var0 IS NOT NULL EXEC(N'ALTER TABLE [People] DROP CONSTRAINT [' + @var0 + '];');
-ALTER TABLE [People] ALTER COLUMN [SomeColumn] nvarchar(max) NOT NULL;");
+ALTER TABLE [People] ALTER COLUMN [SomeColumn] nvarchar(max) NOT NULL;
+ALTER TABLE [People] ADD DEFAULT N'' FOR [SomeColumn];");
         }
 
         [ConditionalFact]
@@ -519,6 +520,7 @@ FROM [sys].[default_constraints] [d]
 WHERE ([d].[parent_object_id] = OBJECT_ID(N'[People]') AND [c].[name] = N'SomeColumn');
 IF @var0 IS NOT NULL EXEC(N'ALTER TABLE [People] DROP CONSTRAINT [' + @var0 + '];');
 ALTER TABLE [People] ALTER COLUMN [SomeColumn] nvarchar(450) NOT NULL;
+ALTER TABLE [People] ADD DEFAULT N'' FOR [SomeColumn];
 CREATE INDEX [IX_People_SomeColumn] ON [People] ([SomeColumn]);");
         }
 
@@ -536,6 +538,7 @@ FROM [sys].[default_constraints] [d]
 WHERE ([d].[parent_object_id] = OBJECT_ID(N'[People]') AND [c].[name] = N'FirstName');
 IF @var0 IS NOT NULL EXEC(N'ALTER TABLE [People] DROP CONSTRAINT [' + @var0 + '];');
 ALTER TABLE [People] ALTER COLUMN [FirstName] nvarchar(450) NOT NULL;
+ALTER TABLE [People] ADD DEFAULT N'' FOR [FirstName];
 CREATE INDEX [IX_People_FirstName_LastName] ON [People] ([FirstName], [LastName]);");
         }
 
@@ -661,6 +664,37 @@ FROM [sys].[default_constraints] [d]
 ALTER TABLE [People] ALTER COLUMN [Name] nvarchar(max) COLLATE German_PhoneBook_CI_AS NULL;");
         }
 
+        [ConditionalFact]
+        public virtual async Task Alter_column_set_collation_with_index()
+        {
+            await Test(
+                builder => builder.Entity(
+                    "People", e =>
+                    {
+                        e.Property<string>("Name");
+                        e.HasIndex("Name");
+                    }),
+                builder => { },
+                builder => builder.Entity("People").Property<string>("Name")
+                    .UseCollation(NonDefaultCollation),
+                model =>
+                {
+                    var nameColumn = Assert.Single(Assert.Single(model.Tables).Columns);
+                    Assert.Equal(NonDefaultCollation, nameColumn.Collation);
+                });
+
+            AssertSql(
+                @"DROP INDEX [IX_People_Name] ON [People];
+DECLARE @var0 sysname;
+SELECT @var0 = [d].[name]
+FROM [sys].[default_constraints] [d]
+INNER JOIN [sys].[columns] [c] ON [d].[parent_column_id] = [c].[column_id] AND [d].[parent_object_id] = [c].[object_id]
+WHERE ([d].[parent_object_id] = OBJECT_ID(N'[People]') AND [c].[name] = N'Name');
+IF @var0 IS NOT NULL EXEC(N'ALTER TABLE [People] DROP CONSTRAINT [' + @var0 + '];');
+ALTER TABLE [People] ALTER COLUMN [Name] nvarchar(450) COLLATE German_PhoneBook_CI_AS NULL;
+CREATE INDEX [IX_People_Name] ON [People] ([Name]);");
+        }
+
         [ConditionalFact]
         public override async Task Alter_column_reset_collation()
         {
@@ -711,6 +745,7 @@ FROM [sys].[default_constraints] [d]
 WHERE ([d].[parent_object_id] = OBJECT_ID(N'[People]') AND [c].[name] = N'SomeColumn');
 IF @var0 IS NOT NULL EXEC(N'ALTER TABLE [People] DROP CONSTRAINT [' + @var0 + '];');
 ALTER TABLE [People] ALTER COLUMN [SomeColumn] nvarchar(450) NOT NULL;
+ALTER TABLE [People] ADD DEFAULT N'' FOR [SomeColumn];
 CREATE INDEX [IX_People_SomeColumn] ON [People] ([SomeColumn]) INCLUDE ([SomeOtherColumn]);");
         }
 
@@ -1470,7 +1505,7 @@ public override async Task Add_primary_key_composite_with_name()
         public virtual async Task Add_primary_key_nonclustered()
         {
             await Test(
-                builder => builder.Entity("People").Property<string>("SomeField"),
+                builder => builder.Entity("People").Property<string>("SomeField").IsRequired().HasMaxLength(450),
                 builder => { },
                 builder => builder.Entity("People").HasKey("SomeField").IsClustered(false),
                 model =>
@@ -1482,14 +1517,6 @@ await Test(
                 });
 
             AssertSql(
-                @"DECLARE @var0 sysname;
-SELECT @var0 = [d].[name]
-FROM [sys].[default_constraints] [d]
-INNER JOIN [sys].[columns] [c] ON [d].[parent_column_id] = [c].[column_id] AND [d].[parent_object_id] = [c].[object_id]
-WHERE ([d].[parent_object_id] = OBJECT_ID(N'[People]') AND [c].[name] = N'SomeField');
-IF @var0 IS NOT NULL EXEC(N'ALTER TABLE [People] DROP CONSTRAINT [' + @var0 + '];');
-ALTER TABLE [People] ALTER COLUMN [SomeField] nvarchar(450) NOT NULL;",
-                //
                 @"ALTER TABLE [People] ADD CONSTRAINT [PK_People] PRIMARY KEY NONCLUSTERED ([SomeField]);");
         }