EF Core Patterns

Reference material for EF Core development.

Migration Safety

Safe Operations

• Adding a new table
• Adding a nullable column
• Adding a column with a default value
• Adding an index
• Renaming (with migrationBuilder.RenameColumn — preserves data)

Dangerous Operations (require confirmation)

• Dropping a column (data loss)
• Changing column type (potential data truncation)
• Adding NOT NULL column without default (fails on existing rows)
• Dropping a table (data loss)
• Removing an index (performance impact)

Migration Best Practices

• One migration per logical change
• Generate with dotnet ef migrations add, then review and adjust the generated Up/Down (a rename scaffolded as drop + add must become RenameColumn; data moves go in Sql()). Never hand-write a migration from scratch — the model snapshot would drift
• dotnet ef database update has no dry-run: review the SQL with dotnet ef migrations script <from> <to> (--idempotent for scripts run against unknown state)
• dotnet ef migrations has-pending-model-changes (EF Core 8+) fails when the model has changed since the last migration — use it as a CI gate
• Keep migrations small and reversible
• Use migrationBuilder.Sql() for data migrations, not EF operations

Configuration Patterns

Fluent API (preferred)

public class UserConfiguration : IEntityTypeConfiguration<User>
{
    public void Configure(EntityTypeBuilder<User> builder)
    {
        builder.HasKey(u => u.Id);
        builder.Property(u => u.Email).HasMaxLength(256).IsRequired();
        builder.HasIndex(u => u.Email).IsUnique();
        builder.HasMany(u => u.Orders).WithOne(o => o.User).HasForeignKey(o => o.UserId);
    }
}

DbContext Registration

// In OnModelCreating:
modelBuilder.ApplyConfigurationsFromAssembly(typeof(ApplicationDbContext).Assembly);

Query Optimization

Do

• Use AsNoTracking() for read-only queries
• Use projections (Select) to fetch only needed columns
• Use Include with ThenInclude for eager loading (avoid N+1)
• Use split queries for many includes: .AsSplitQuery()
• Use ExecuteUpdateAsync / ExecuteDeleteAsync for bulk operations (EF 7+)

Don't

• Don't call ToList() before filtering (materializes entire table)
• Don't use Count() when you need Any()
• Don't load navigation properties you don't need
• Don't use FromSqlRaw with string interpolation (SQL injection risk)
• Don't forget CancellationToken on async queries

See Also

• skills/ddd/SKILL.md — value converters and complex properties for DDD entities
• skills/testing-dotnet/SKILL.md — testing queries against the real provider