Generate UUIDv7 values for value-generated strings (#3462)

Fixes #3460

(cherry picked from commit 16841f9)

Author: roji

src/EFCore.PG/ValueGeneration/Internal/NpgsqlValueGeneratorSelector.cs

@@ -101,9 +101,14 @@ public override bool TrySelect(IProperty property, ITypeBase typeBase, out Value
     ///     doing so can result in application failures when updating to a new Entity Framework Core release.
     /// </summary>
     protected override ValueGenerator? FindForType(IProperty property, ITypeBase typeBase, Type clrType)
-        => property.ClrType.UnwrapNullableType() == typeof(Guid)
-            ? property.ValueGenerated == ValueGenerated.Never || property.GetDefaultValueSql() is not null
-                ? new TemporaryGuidValueGenerator()
-                : new NpgsqlSequentialGuidValueGenerator()
-            : base.FindForType(property, typeBase, clrType);
+        => property.ClrType.UnwrapNullableType() switch
+        {
+            var t when t == typeof(Guid) && property.ValueGenerated is not ValueGenerated.Never && property.GetDefaultValueSql() is null
+                => new NpgsqlSequentialGuidValueGenerator(),
+
+            var t when t == typeof(string) && property.ValueGenerated is not ValueGenerated.Never && property.GetDefaultValueSql() is null
+                => new NpgsqlSequentialStringValueGenerator(),
+
+            _ => base.FindForType(property, typeBase, clrType)
+        };
 }

src/EFCore.PG/ValueGeneration/NpgsqlSequentialStringValueGenerator.cs

@@ -0,0 +1,22 @@
+namespace Npgsql.EntityFrameworkCore.PostgreSQL.ValueGeneration;
+
+/// <summary>
+///     This API supports the Entity Framework Core infrastructure and is not intended to be used
+///     directly from your code. This API may change or be removed in future releases.
+/// </summary>
+public class NpgsqlSequentialStringValueGenerator : ValueGenerator<string>
+{
+    private readonly NpgsqlSequentialGuidValueGenerator _guidGenerator = new();
+
+    /// <summary>
+    ///     This API supports the Entity Framework Core infrastructure and is not intended to be used
+    ///     directly from your code. This API may change or be removed in future releases.
+    /// </summary>
+    public override bool GeneratesTemporaryValues => false;
+
+    /// <summary>
+    ///     This API supports the Entity Framework Core infrastructure and is not intended to be used
+    ///     directly from your code. This API may change or be removed in future releases.
+    /// </summary>
+    public override string Next(EntityEntry entry) => _guidGenerator.Next(entry).ToString();
+}

test/EFCore.PG.Tests/NpgsqlValueGeneratorSelectorTest.cs

@@ -21,7 +21,7 @@ public void Returns_built_in_generators_for_types_setup_for_value_generation()
         AssertGenerator<TemporaryShortValueGenerator>("NullableShort");
         AssertGenerator<TemporaryByteValueGenerator>("NullableByte");
         AssertGenerator<TemporaryDecimalValueGenerator>("Decimal");
-        AssertGenerator<StringValueGenerator>("String");
+        AssertGenerator<NpgsqlSequentialStringValueGenerator>("String");
         AssertGenerator<NpgsqlSequentialGuidValueGenerator>("Guid");
         AssertGenerator<BinaryValueGenerator>("Binary");
     }
@@ -128,7 +128,7 @@ public void Returns_sequence_value_generators_when_configured_for_model()
         AssertGenerator<NpgsqlSequenceHiLoValueGenerator<int>>("NullableInt", setSequences: true);
         AssertGenerator<NpgsqlSequenceHiLoValueGenerator<long>>("NullableLong", setSequences: true);
         AssertGenerator<NpgsqlSequenceHiLoValueGenerator<short>>("NullableShort", setSequences: true);
-        AssertGenerator<StringValueGenerator>("String", setSequences: true);
+        AssertGenerator<NpgsqlSequentialStringValueGenerator>("String", setSequences: true);
         AssertGenerator<NpgsqlSequentialGuidValueGenerator>("Guid", setSequences: true);
         AssertGenerator<BinaryValueGenerator>("Binary", setSequences: true);
     }