Fix ComplexCollection ToJson migration default value: use "[]" instead of "{}"

[Copilot]

When adding a ComplexCollection mapped with ToJson() to an existing table, the generated migration incorrectly sets defaultValue: "{}" (JSON object) instead of defaultValue: "[]" (JSON array), causing a runtime InvalidOperationException when reading entities with the default value.

Root Cause

MigrationsModelDiffer.InitializeJsonColumn unconditionally used "{}" as the default for all non-nullable JSON columns, without distinguishing collections from single objects.

Changes

• MigrationsModelDiffer.cs: Added IsJsonCollectionColumn helper that inspects the table's ComplexTypeMappings and EntityTypeMappings to detect collection-backed JSON columns:

  • Complex collections: ComplexProperty.IsCollection == true and GetContainerColumnName() matches the column
  • Owned collections (OwnsMany): ownership IsUnique: false
  • Uses "[]" as default for collection columns, "{}" for single-object columns

• MigrationsModelDifferTest.cs: Added Add_complex_collection_mapped_to_json_uses_empty_array_as_default_value test covering the reported scenario.

// Before fix: defaultValue: "{}"
// After fix:  defaultValue: "[]"
migrationBuilder.AddColumn<string>(
    name: "Items",
    table: "MyEntities",
    type: "nvarchar(max)",
    nullable: false,
    defaultValue: "[]");

Original prompt

---

This section details on the original issue you should resolve

<issue_title>ComplexCollection ToJson produces a {} default value instead of [] in migrations</issue_title>
<issue_description>### Bug description

• Add a property e.ComplexCollection(m => m.MyCollection, x => x.ToJson()); to an existing entity.
• Generate migrations (SQL Server, compat 170)
• Observe that defaultValue in the migration is {} instead of [].

Your code

// ComplexCollection with ToJson() generates defaultValue: "{}" instead of "[]"
// when adding the column to an existing table.
//
// Steps to reproduce:
//   1. Comment out Items property and ComplexCollection config.
//   2. dotnet ef migrations add Init        (creates the table without Items)
//   3. Uncomment Items property and ComplexCollection config.
//   4. dotnet ef migrations add AddItems    (adds Items column to existing table)
//   5. Inspect Migrations/*_AddItems.cs — the Items column has defaultValue: "{}" instead of "[]".

using System.Collections.Generic;
using Microsoft.EntityFrameworkCore;

var db = new BugReproContext();

public class MyEntity
{
    public int Id { get; set; }
    public List<MyComplexType> Items { get; set; } = [];
}

public class MyComplexType
{
    public string Name { get; set; } = "";
    public int Value { get; set; }
}

public class BugReproContext : DbContext
{
    public DbSet<MyEntity> MyEntities { get; set; }

    protected override void OnConfiguring(DbContextOptionsBuilder options)
        => options.UseSqlServer("Server=(localdb)\\mssqllocaldb;Database=EfCoreBugRepro;Trusted_Connection=True;",
            o => o.UseCompatibilityLevel(170));

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        modelBuilder.Entity<MyEntity>(e =>
        {
            e.ComplexCollection(m => m.Items, x => x.ToJson());
        });
    }
}

Stack traces

Attempts to read the entity after the incorrect migration runs results in the following due to JSON objects being present where JSON arrays are expected:

An exception of type 'System.InvalidOperationException' occurred in System.Private.CoreLib.dll but was not handled in user code: 'Invalid token type: 'StartObject'.'
   at Microsoft.EntityFrameworkCore.Query.RelationalShapedQueryCompilingExpressionVisitor.ShaperProcessingExpressionVisitor.IncludeJsonEntityCollection[TIncludingEntity,TIncludedCollectionElement](QueryContext queryContext, Object[] keyPropertyValues, JsonReaderData jsonReaderData, TIncludingEntity entity, Func`4 innerShaper, Action`1 getOrCreateCollectionObject, Action`2 fixup, Boolean performFixup)
   at Microsoft.EntityFrameworkCore.Query.Internal.SplitQueryingEnumerable`1.AsyncEnumerator.<MoveNextAsync>d__21.MoveNext()
   at System.Runtime.CompilerServices.ConfiguredValueTaskAwaitable`1.ConfiguredValueTaskAwaiter.GetResult()
   at Microsoft.EntityFrameworkCore.EntityFrameworkQueryableExtensions.<ToListAsync>d__67`1.MoveNext()
   at Microsoft.EntityFrameworkCore.EntityFrameworkQueryableExtensions.<ToListAsync>d__67`1.MoveNext()

Verbose output

public partial class AddItems : Migration
    {
        /// <inheritdoc />
        protected override void Up(MigrationBuilder migrationBuilder)
        {
            migrationBuilder.AddColumn<string>(
                name: "Items",
                table: "MyEntities",
                type: "json",
                nullable: false,
                defaultValue: "{}");
        }

        /// <inheritdoc />
        protected override void Down(MigrationBuilder migrationBuilder)
        {
            migrationBuilder.DropColumn(
                name: "Items",
                table: "MyEntities");
        }
    }

EF Core version

10.0.5

Database provider

Microsoft.EntityFrameworkCore.SqlServer

Target framework

.NET 10

Operating system

Windows 11

IDE

VS Code</issue_description>

Comments on the Issue (you are @copilot in this section)

@roji # AI Triage

The below is an AI-generated analysis and may contain inaccuracies.

Summary

Confirmed bug. When adding a ComplexCollection mapped with ToJson() to an existing entity via migrations, the generated migration sets defaultValue: "{}" (JSON object) instead of defaultValue: "[]" (JSON array). This causes a runtime InvalidOperationException ("Invalid token type: 'StartObject'") when reading entities with the default value.

Root Cause

In MigrationsModelDiffer.InitializeJsonColumn, the default value is unconditionally set to "{}" for all non-nullable, non-inline JSON columns:

columnOperation.DefaultValue = inline || isNullable
    ? null
    : "{}";

This does not distinguish between complex properties (JSON objects → "{}" is correct) and complex collections (JS...

• Fixes ComplexCollection ToJson produces a {} default value instead of [] in migrations #37964

---

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[roji]

Add a test covering the owned (non-complex) case, which you seem to have fixed as well.

[Copilot]

Pull request overview

Fixes migration generation for ComplexCollection(...).ToJson() so non-nullable JSON collection columns get an empty JSON array default ("[]") rather than an empty JSON object ("{}"), preventing runtime failures when materializing entities from the default value.

Changes:

• Update MigrationsModelDiffer.InitializeJsonColumn to choose "[]" vs "{}" based on whether the JSON column represents a collection.
• Add IsJsonCollectionColumn(JsonColumn) helper to detect collection-backed JSON container columns.
• Add migration-diff tests covering the complex-collection scenario (and additional JSON owned-type scenarios).

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
src/EFCore.Relational/Migrations/Internal/MigrationsModelDiffer.cs	Adjusts default JSON value generation for non-nullable JSON container columns, introducing collection detection.
test/EFCore.Relational.Tests/Migrations/Internal/MigrationsModelDifferTest.cs	Adds regression coverage asserting "[]" default for complex collections mapped to JSON and validating behaviors for owned JSON mappings.