Fix ComplexCollection ToJson default value in migrations to use [] instead of {}

Co-authored-by: roji <1862641+roji@users.noreply.github.com>
Agent-Logs-Url: https://github.com/dotnet/efcore/sessions/1fd85ba2-cb75-4e30-b1f5-afbf64cc4626

Author: Copilot

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

@@ -1279,11 +1279,21 @@ private void InitializeJsonColumn(
         columnOperation.ClrType = typeof(string);
         columnOperation.DefaultValue = inline || isNullable
             ? null
-            : "{}";
+            : IsJsonCollectionColumn(jsonColumn) ? "[]" : "{}";
 
         columnOperation.AddAnnotations(migrationsAnnotations);
     }
 
+    private static bool IsJsonCollectionColumn(JsonColumn jsonColumn)
+        => jsonColumn.Table.ComplexTypeMappings.Any(
+               m => m.TypeBase is IComplexType ct
+                   && ct.GetContainerColumnName() == jsonColumn.Name
+                   && ct.ComplexProperty.IsCollection)
+           || jsonColumn.Table.EntityTypeMappings.Any(
+               m => m.TypeBase is IEntityType et
+                   && et.GetContainerColumnName() == jsonColumn.Name
+                   && et.FindOwnership() is { IsUnique: false });
+
     #endregion
 
     #region IKey

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

@@ -10007,6 +10007,52 @@ public virtual void Convert_table_from_owned_to_complex_properties_mapped_to_jso
             Assert.Empty);
 #pragma warning restore EF8001 // Owned JSON entities are obsolete
 
+    [ConditionalFact]
+    public virtual void Add_complex_collection_mapped_to_json_uses_empty_array_as_default_value()
+        => Execute(
+            _ => { },
+            source =>
+            {
+                source.Entity(
+                    "Entity", e =>
+                    {
+                        e.Property<int>("Id").ValueGeneratedOnAdd();
+                        e.HasKey("Id");
+                    });
+            },
+            target =>
+            {
+                target.Entity(
+                    "Entity", e =>
+                    {
+                        e.Property<int>("Id").ValueGeneratedOnAdd();
+                        e.HasKey("Id");
+
+                        e.ComplexCollection<List<MyJsonComplex>, MyJsonComplex>(
+                            "ComplexCollection", cp =>
+                            {
+                                cp.IsRequired();
+                                cp.ToJson("json_collection");
+                                cp.Property(x => x.Value);
+                                cp.Property(x => x.Date);
+                            });
+                    });
+            },
+            upOps =>
+            {
+                Assert.Equal(1, upOps.Count);
+
+                var operation = Assert.IsType<AddColumnOperation>(upOps[0]);
+                Assert.Equal("Entity", operation.Table);
+                Assert.Equal("json_collection", operation.Name);
+                Assert.Equal("[]", operation.DefaultValue);
+            },
+            downOps =>
+            {
+                Assert.Equal(1, downOps.Count);
+                Assert.IsType<DropColumnOperation>(downOps[0]);
+            });
+
     [ConditionalFact]
     public virtual void Noop_on_complex_properties()
         => Execute(