From 5925aa796aac20d4f275af444e65e10b162ca23f Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Marek=20Jasi=C5=84ski?= <jasins.marek@gmail.com>
Date: Wed, 13 May 2026 20:51:25 +0200
Subject: [PATCH] fix: make EbookId nullable to support global knowledge units
 and resolve PR comments

---
 .../Migrations/AppDbContextModelSnapshot.cs   |   5 +-
 ...keKnowledgeUnitEbookIdNullable.Designer.cs | 711 ++++++++++++++++++
 ...185108_MakeKnowledgeUnitEbookIdNullable.cs |  37 +
 .../Entities/KnowledgeUnit.cs                 |   9 +-
 .../Services/KnowledgeService.cs              |   6 +-
 5 files changed, 756 insertions(+), 12 deletions(-)
 create mode 100644 src/NexusReader.Data/Persistence/Migrations/20260513185108_MakeKnowledgeUnitEbookIdNullable.Designer.cs
 create mode 100644 src/NexusReader.Data/Persistence/Migrations/20260513185108_MakeKnowledgeUnitEbookIdNullable.cs

diff --git a/src/NexusReader.Data/Migrations/AppDbContextModelSnapshot.cs b/src/NexusReader.Data/Migrations/AppDbContextModelSnapshot.cs
index 8bf5f92..9da5042 100644
--- a/src/NexusReader.Data/Migrations/AppDbContextModelSnapshot.cs
+++ b/src/NexusReader.Data/Migrations/AppDbContextModelSnapshot.cs
@@ -247,7 +247,7 @@ namespace NexusReader.Data.Migrations
                     b.Property<DateTime>("CreatedAt")
                         .HasColumnType("timestamp with time zone");
 
-                    b.Property<Guid>("EbookId")
+                    b.Property<Guid?>("EbookId")
                         .HasColumnType("uuid");
 
                     b.Property<string>("MetadataJson")
@@ -638,8 +638,7 @@ namespace NexusReader.Data.Migrations
                     b.HasOne("NexusReader.Domain.Entities.Ebook", "Ebook")
                         .WithMany()
                         .HasForeignKey("EbookId")
-                        .OnDelete(DeleteBehavior.Cascade)
-                        .IsRequired();
+                        .OnDelete(DeleteBehavior.Cascade);
 
                     b.Navigation("Ebook");
                 });
diff --git a/src/NexusReader.Data/Persistence/Migrations/20260513185108_MakeKnowledgeUnitEbookIdNullable.Designer.cs b/src/NexusReader.Data/Persistence/Migrations/20260513185108_MakeKnowledgeUnitEbookIdNullable.Designer.cs
new file mode 100644
index 0000000..fb31954
--- /dev/null
+++ b/src/NexusReader.Data/Persistence/Migrations/20260513185108_MakeKnowledgeUnitEbookIdNullable.Designer.cs
@@ -0,0 +1,711 @@
+// <auto-generated />
+using System;
+using Microsoft.EntityFrameworkCore;
+using Microsoft.EntityFrameworkCore.Infrastructure;
+using Microsoft.EntityFrameworkCore.Migrations;
+using Microsoft.EntityFrameworkCore.Storage.ValueConversion;
+using NexusReader.Data.Persistence;
+using Npgsql.EntityFrameworkCore.PostgreSQL.Metadata;
+using Pgvector;
+
+#nullable disable
+
+namespace NexusReader.Data.Persistence.Migrations
+{
+    [DbContext(typeof(AppDbContext))]
+    [Migration("20260513185108_MakeKnowledgeUnitEbookIdNullable")]
+    partial class MakeKnowledgeUnitEbookIdNullable
+    {
+        /// <inheritdoc />
+        protected override void BuildTargetModel(ModelBuilder modelBuilder)
+        {
+#pragma warning disable 612, 618
+            modelBuilder
+                .HasAnnotation("ProductVersion", "10.0.7")
+                .HasAnnotation("Relational:MaxIdentifierLength", 63);
+
+            NpgsqlModelBuilderExtensions.HasPostgresExtension(modelBuilder, "vector");
+            NpgsqlModelBuilderExtensions.UseIdentityByDefaultColumns(modelBuilder);
+
+            modelBuilder.Entity("Microsoft.AspNetCore.Identity.IdentityRole", b =>
+                {
+                    b.Property<string>("Id")
+                        .HasColumnType("text");
+
+                    b.Property<string>("ConcurrencyStamp")
+                        .IsConcurrencyToken()
+                        .HasColumnType("text");
+
+                    b.Property<string>("Name")
+                        .HasMaxLength(256)
+                        .HasColumnType("character varying(256)");
+
+                    b.Property<string>("NormalizedName")
+                        .HasMaxLength(256)
+                        .HasColumnType("character varying(256)");
+
+                    b.HasKey("Id");
+
+                    b.HasIndex("NormalizedName")
+                        .IsUnique()
+                        .HasDatabaseName("RoleNameIndex");
+
+                    b.ToTable("AspNetRoles", (string)null);
+                });
+
+            modelBuilder.Entity("Microsoft.AspNetCore.Identity.IdentityRoleClaim<string>", b =>
+                {
+                    b.Property<int>("Id")
+                        .ValueGeneratedOnAdd()
+                        .HasColumnType("integer");
+
+                    NpgsqlPropertyBuilderExtensions.UseIdentityByDefaultColumn(b.Property<int>("Id"));
+
+                    b.Property<string>("ClaimType")
+                        .HasColumnType("text");
+
+                    b.Property<string>("ClaimValue")
+                        .HasColumnType("text");
+
+                    b.Property<string>("RoleId")
+                        .IsRequired()
+                        .HasColumnType("text");
+
+                    b.HasKey("Id");
+
+                    b.HasIndex("RoleId");
+
+                    b.ToTable("AspNetRoleClaims", (string)null);
+                });
+
+            modelBuilder.Entity("Microsoft.AspNetCore.Identity.IdentityUserClaim<string>", b =>
+                {
+                    b.Property<int>("Id")
+                        .ValueGeneratedOnAdd()
+                        .HasColumnType("integer");
+
+                    NpgsqlPropertyBuilderExtensions.UseIdentityByDefaultColumn(b.Property<int>("Id"));
+
+                    b.Property<string>("ClaimType")
+                        .HasColumnType("text");
+
+                    b.Property<string>("ClaimValue")
+                        .HasColumnType("text");
+
+                    b.Property<string>("UserId")
+                        .IsRequired()
+                        .HasColumnType("text");
+
+                    b.HasKey("Id");
+
+                    b.HasIndex("UserId");
+
+                    b.ToTable("AspNetUserClaims", (string)null);
+                });
+
+            modelBuilder.Entity("Microsoft.AspNetCore.Identity.IdentityUserLogin<string>", b =>
+                {
+                    b.Property<string>("LoginProvider")
+                        .HasColumnType("text");
+
+                    b.Property<string>("ProviderKey")
+                        .HasColumnType("text");
+
+                    b.Property<string>("ProviderDisplayName")
+                        .HasColumnType("text");
+
+                    b.Property<string>("UserId")
+                        .IsRequired()
+                        .HasColumnType("text");
+
+                    b.HasKey("LoginProvider", "ProviderKey");
+
+                    b.HasIndex("UserId");
+
+                    b.ToTable("AspNetUserLogins", (string)null);
+                });
+
+            modelBuilder.Entity("Microsoft.AspNetCore.Identity.IdentityUserRole<string>", b =>
+                {
+                    b.Property<string>("UserId")
+                        .HasColumnType("text");
+
+                    b.Property<string>("RoleId")
+                        .HasColumnType("text");
+
+                    b.HasKey("UserId", "RoleId");
+
+                    b.HasIndex("RoleId");
+
+                    b.ToTable("AspNetUserRoles", (string)null);
+                });
+
+            modelBuilder.Entity("Microsoft.AspNetCore.Identity.IdentityUserToken<string>", b =>
+                {
+                    b.Property<string>("UserId")
+                        .HasColumnType("text");
+
+                    b.Property<string>("LoginProvider")
+                        .HasColumnType("text");
+
+                    b.Property<string>("Name")
+                        .HasColumnType("text");
+
+                    b.Property<string>("Value")
+                        .HasColumnType("text");
+
+                    b.HasKey("UserId", "LoginProvider", "Name");
+
+                    b.ToTable("AspNetUserTokens", (string)null);
+                });
+
+            modelBuilder.Entity("NexusReader.Domain.Entities.Author", b =>
+                {
+                    b.Property<int>("Id")
+                        .ValueGeneratedOnAdd()
+                        .HasColumnType("integer");
+
+                    NpgsqlPropertyBuilderExtensions.UseIdentityByDefaultColumn(b.Property<int>("Id"));
+
+                    b.Property<string>("Name")
+                        .IsRequired()
+                        .HasMaxLength(255)
+                        .HasColumnType("character varying(255)");
+
+                    b.HasKey("Id");
+
+                    b.ToTable("Authors");
+                });
+
+            modelBuilder.Entity("NexusReader.Domain.Entities.Ebook", b =>
+                {
+                    b.Property<Guid>("Id")
+                        .ValueGeneratedOnAdd()
+                        .HasColumnType("uuid");
+
+                    b.Property<DateTime>("AddedDate")
+                        .HasColumnType("timestamp with time zone");
+
+                    b.Property<int>("AuthorId")
+                        .HasColumnType("integer");
+
+                    b.Property<string>("CoverUrl")
+                        .HasColumnType("text");
+
+                    b.Property<string>("FilePath")
+                        .IsRequired()
+                        .HasColumnType("text");
+
+                    b.Property<bool>("IsReadyForReading")
+                        .HasColumnType("boolean");
+
+                    b.Property<string>("LastChapter")
+                        .HasMaxLength(255)
+                        .HasColumnType("character varying(255)");
+
+                    b.Property<int>("LastChapterIndex")
+                        .HasColumnType("integer");
+
+                    b.Property<DateTime?>("LastReadDate")
+                        .HasColumnType("timestamp with time zone");
+
+                    b.Property<double>("Progress")
+                        .HasColumnType("double precision");
+
+                    b.Property<string>("TenantId")
+                        .IsRequired()
+                        .HasMaxLength(128)
+                        .HasColumnType("character varying(128)");
+
+                    b.Property<string>("Title")
+                        .IsRequired()
+                        .HasMaxLength(255)
+                        .HasColumnType("character varying(255)");
+
+                    b.Property<string>("UserId")
+                        .IsRequired()
+                        .HasColumnType("text");
+
+                    b.HasKey("Id");
+
+                    b.HasIndex("AuthorId");
+
+                    b.HasIndex("TenantId");
+
+                    b.HasIndex("UserId");
+
+                    b.ToTable("Ebooks");
+                });
+
+            modelBuilder.Entity("NexusReader.Domain.Entities.KnowledgeUnit", b =>
+                {
+                    b.Property<string>("Id")
+                        .HasMaxLength(128)
+                        .HasColumnType("character varying(128)");
+
+                    b.Property<string>("Content")
+                        .IsRequired()
+                        .HasColumnType("text");
+
+                    b.Property<DateTime>("CreatedAt")
+                        .HasColumnType("timestamp with time zone");
+
+                    b.Property<Guid?>("EbookId")
+                        .HasColumnType("uuid");
+
+                    b.Property<string>("MetadataJson")
+                        .HasColumnType("text");
+
+                    b.Property<string>("TenantId")
+                        .IsRequired()
+                        .HasMaxLength(128)
+                        .HasColumnType("character varying(128)");
+
+                    b.Property<int>("Type")
+                        .HasColumnType("integer");
+
+                    b.Property<Vector>("Vector")
+                        .HasColumnType("vector(768)");
+
+                    b.Property<string>("Version")
+                        .IsRequired()
+                        .HasMaxLength(50)
+                        .HasColumnType("character varying(50)");
+
+                    b.HasKey("Id");
+
+                    b.HasIndex("EbookId");
+
+                    b.HasIndex("TenantId");
+
+                    b.ToTable("KnowledgeUnits");
+                });
+
+            modelBuilder.Entity("NexusReader.Domain.Entities.KnowledgeUnitLink", b =>
+                {
+                    b.Property<int>("Id")
+                        .ValueGeneratedOnAdd()
+                        .HasColumnType("integer");
+
+                    NpgsqlPropertyBuilderExtensions.UseIdentityByDefaultColumn(b.Property<int>("Id"));
+
+                    b.Property<string>("RelationType")
+                        .IsRequired()
+                        .HasMaxLength(50)
+                        .HasColumnType("character varying(50)");
+
+                    b.Property<string>("SourceUnitId")
+                        .IsRequired()
+                        .HasMaxLength(128)
+                        .HasColumnType("character varying(128)");
+
+                    b.Property<string>("TargetUnitId")
+                        .IsRequired()
+                        .HasMaxLength(128)
+                        .HasColumnType("character varying(128)");
+
+                    b.HasKey("Id");
+
+                    b.HasIndex("SourceUnitId");
+
+                    b.HasIndex("TargetUnitId");
+
+                    b.ToTable("KnowledgeUnitLinks");
+                });
+
+            modelBuilder.Entity("NexusReader.Domain.Entities.NexusUser", b =>
+                {
+                    b.Property<string>("Id")
+                        .HasColumnType("text");
+
+                    b.Property<int>("AITokenLimit")
+                        .HasColumnType("integer");
+
+                    b.Property<int>("AITokensUsed")
+                        .HasColumnType("integer");
+
+                    b.Property<int>("AccessFailedCount")
+                        .HasColumnType("integer");
+
+                    b.Property<string>("ConcurrencyStamp")
+                        .IsConcurrencyToken()
+                        .HasColumnType("text");
+
+                    b.Property<string>("DisplayName")
+                        .HasMaxLength(100)
+                        .HasColumnType("character varying(100)");
+
+                    b.Property<string>("Email")
+                        .HasMaxLength(256)
+                        .HasColumnType("character varying(256)");
+
+                    b.Property<bool>("EmailConfirmed")
+                        .HasColumnType("boolean");
+
+                    b.Property<DateTime?>("LastAiActionDate")
+                        .HasColumnType("timestamp with time zone");
+
+                    b.Property<DateTime?>("LastReadAt")
+                        .HasColumnType("timestamp with time zone");
+
+                    b.Property<string>("LastReadPageId")
+                        .HasMaxLength(255)
+                        .HasColumnType("character varying(255)");
+
+                    b.Property<bool>("LockoutEnabled")
+                        .HasColumnType("boolean");
+
+                    b.Property<DateTimeOffset?>("LockoutEnd")
+                        .HasColumnType("timestamp with time zone");
+
+                    b.Property<string>("NormalizedEmail")
+                        .HasMaxLength(256)
+                        .HasColumnType("character varying(256)");
+
+                    b.Property<string>("NormalizedUserName")
+                        .HasMaxLength(256)
+                        .HasColumnType("character varying(256)");
+
+                    b.Property<string>("PasswordHash")
+                        .HasColumnType("text");
+
+                    b.Property<string>("PhoneNumber")
+                        .HasColumnType("text");
+
+                    b.Property<bool>("PhoneNumberConfirmed")
+                        .HasColumnType("boolean");
+
+                    b.Property<string>("SecurityStamp")
+                        .HasColumnType("text");
+
+                    b.Property<int>("SubscriptionPlanId")
+                        .ValueGeneratedOnAdd()
+                        .HasColumnType("integer")
+                        .HasDefaultValue(1);
+
+                    b.Property<string>("TenantId")
+                        .IsRequired()
+                        .HasMaxLength(128)
+                        .HasColumnType("character varying(128)");
+
+                    b.Property<bool>("TwoFactorEnabled")
+                        .HasColumnType("boolean");
+
+                    b.Property<string>("UserName")
+                        .HasMaxLength(256)
+                        .HasColumnType("character varying(256)");
+
+                    b.HasKey("Id");
+
+                    b.HasIndex("NormalizedEmail")
+                        .HasDatabaseName("EmailIndex");
+
+                    b.HasIndex("NormalizedUserName")
+                        .IsUnique()
+                        .HasDatabaseName("UserNameIndex");
+
+                    b.HasIndex("SubscriptionPlanId");
+
+                    b.HasIndex("TenantId");
+
+                    b.ToTable("AspNetUsers", (string)null);
+                });
+
+            modelBuilder.Entity("NexusReader.Domain.Entities.QuizResult", b =>
+                {
+                    b.Property<Guid>("Id")
+                        .ValueGeneratedOnAdd()
+                        .HasColumnType("uuid");
+
+                    b.Property<DateTime>("CompletedDate")
+                        .HasColumnType("timestamp with time zone");
+
+                    b.Property<int>("Score")
+                        .HasColumnType("integer");
+
+                    b.Property<string>("TenantId")
+                        .IsRequired()
+                        .HasMaxLength(128)
+                        .HasColumnType("character varying(128)");
+
+                    b.Property<string>("Topic")
+                        .IsRequired()
+                        .HasColumnType("text");
+
+                    b.Property<int>("TotalQuestions")
+                        .HasColumnType("integer");
+
+                    b.Property<string>("UserId")
+                        .IsRequired()
+                        .HasColumnType("text");
+
+                    b.HasKey("Id");
+
+                    b.HasIndex("TenantId");
+
+                    b.HasIndex("UserId");
+
+                    b.ToTable("QuizResults");
+                });
+
+            modelBuilder.Entity("NexusReader.Domain.Entities.SemanticKnowledgeCache", b =>
+                {
+                    b.Property<string>("ContentHash")
+                        .HasMaxLength(128)
+                        .HasColumnType("character varying(128)");
+
+                    b.Property<DateTime>("CreatedAt")
+                        .HasColumnType("timestamp with time zone");
+
+                    b.Property<string>("JsonData")
+                        .IsRequired()
+                        .HasColumnType("text");
+
+                    b.Property<string>("ModelId")
+                        .IsRequired()
+                        .HasMaxLength(50)
+                        .HasColumnType("character varying(50)");
+
+                    b.Property<string>("OriginalText")
+                        .IsRequired()
+                        .HasColumnType("text");
+
+                    b.Property<string>("PromptVersion")
+                        .IsRequired()
+                        .HasMaxLength(10)
+                        .HasColumnType("character varying(10)");
+
+                    b.Property<string>("TenantId")
+                        .IsRequired()
+                        .HasMaxLength(128)
+                        .HasColumnType("character varying(128)");
+
+                    b.Property<Vector>("Vector")
+                        .HasColumnType("vector(1536)");
+
+                    b.HasKey("ContentHash");
+
+                    b.HasIndex("ContentHash")
+                        .IsUnique();
+
+                    b.HasIndex("TenantId");
+
+                    b.ToTable("SemanticKnowledgeCache");
+                });
+
+            modelBuilder.Entity("NexusReader.Domain.Entities.SubscriptionPlan", b =>
+                {
+                    b.Property<int>("Id")
+                        .ValueGeneratedOnAdd()
+                        .HasColumnType("integer");
+
+                    NpgsqlPropertyBuilderExtensions.UseIdentityByDefaultColumn(b.Property<int>("Id"));
+
+                    b.Property<int>("AITokenLimit")
+                        .HasColumnType("integer");
+
+                    b.Property<bool>("IsUnlimitedTokens")
+                        .HasColumnType("boolean");
+
+                    b.Property<decimal>("MonthlyPrice")
+                        .HasColumnType("numeric");
+
+                    b.Property<string>("PlanName")
+                        .IsRequired()
+                        .HasMaxLength(50)
+                        .HasColumnType("character varying(50)");
+
+                    b.Property<string>("StripeProductId")
+                        .IsRequired()
+                        .HasMaxLength(50)
+                        .HasColumnType("character varying(50)");
+
+                    b.HasKey("Id");
+
+                    b.HasIndex("PlanName")
+                        .IsUnique();
+
+                    b.ToTable("SubscriptionPlans");
+
+                    b.HasData(
+                        new
+                        {
+                            Id = 1,
+                            AITokenLimit = 5000,
+                            IsUnlimitedTokens = false,
+                            MonthlyPrice = 0m,
+                            PlanName = "Free",
+                            StripeProductId = "prod_Free789"
+                        },
+                        new
+                        {
+                            Id = 2,
+                            AITokenLimit = 10000,
+                            IsUnlimitedTokens = false,
+                            MonthlyPrice = 9.99m,
+                            PlanName = "Basic",
+                            StripeProductId = "prod_basic_placeholder"
+                        },
+                        new
+                        {
+                            Id = 3,
+                            AITokenLimit = 50000,
+                            IsUnlimitedTokens = false,
+                            MonthlyPrice = 19.99m,
+                            PlanName = "Pro",
+                            StripeProductId = "prod_pro_placeholder"
+                        },
+                        new
+                        {
+                            Id = 4,
+                            AITokenLimit = 1000000000,
+                            IsUnlimitedTokens = true,
+                            MonthlyPrice = 99.99m,
+                            PlanName = "Enterprise",
+                            StripeProductId = "prod_enterprise_placeholder"
+                        });
+                });
+
+            modelBuilder.Entity("Microsoft.AspNetCore.Identity.IdentityRoleClaim<string>", b =>
+                {
+                    b.HasOne("Microsoft.AspNetCore.Identity.IdentityRole", null)
+                        .WithMany()
+                        .HasForeignKey("RoleId")
+                        .OnDelete(DeleteBehavior.Cascade)
+                        .IsRequired();
+                });
+
+            modelBuilder.Entity("Microsoft.AspNetCore.Identity.IdentityUserClaim<string>", b =>
+                {
+                    b.HasOne("NexusReader.Domain.Entities.NexusUser", null)
+                        .WithMany()
+                        .HasForeignKey("UserId")
+                        .OnDelete(DeleteBehavior.Cascade)
+                        .IsRequired();
+                });
+
+            modelBuilder.Entity("Microsoft.AspNetCore.Identity.IdentityUserLogin<string>", b =>
+                {
+                    b.HasOne("NexusReader.Domain.Entities.NexusUser", null)
+                        .WithMany()
+                        .HasForeignKey("UserId")
+                        .OnDelete(DeleteBehavior.Cascade)
+                        .IsRequired();
+                });
+
+            modelBuilder.Entity("Microsoft.AspNetCore.Identity.IdentityUserRole<string>", b =>
+                {
+                    b.HasOne("Microsoft.AspNetCore.Identity.IdentityRole", null)
+                        .WithMany()
+                        .HasForeignKey("RoleId")
+                        .OnDelete(DeleteBehavior.Cascade)
+                        .IsRequired();
+
+                    b.HasOne("NexusReader.Domain.Entities.NexusUser", null)
+                        .WithMany()
+                        .HasForeignKey("UserId")
+                        .OnDelete(DeleteBehavior.Cascade)
+                        .IsRequired();
+                });
+
+            modelBuilder.Entity("Microsoft.AspNetCore.Identity.IdentityUserToken<string>", b =>
+                {
+                    b.HasOne("NexusReader.Domain.Entities.NexusUser", null)
+                        .WithMany()
+                        .HasForeignKey("UserId")
+                        .OnDelete(DeleteBehavior.Cascade)
+                        .IsRequired();
+                });
+
+            modelBuilder.Entity("NexusReader.Domain.Entities.Ebook", b =>
+                {
+                    b.HasOne("NexusReader.Domain.Entities.Author", "Author")
+                        .WithMany("Ebooks")
+                        .HasForeignKey("AuthorId")
+                        .OnDelete(DeleteBehavior.Restrict)
+                        .IsRequired();
+
+                    b.HasOne("NexusReader.Domain.Entities.NexusUser", "User")
+                        .WithMany("Ebooks")
+                        .HasForeignKey("UserId")
+                        .OnDelete(DeleteBehavior.Cascade)
+                        .IsRequired();
+
+                    b.Navigation("Author");
+
+                    b.Navigation("User");
+                });
+
+            modelBuilder.Entity("NexusReader.Domain.Entities.KnowledgeUnit", b =>
+                {
+                    b.HasOne("NexusReader.Domain.Entities.Ebook", "Ebook")
+                        .WithMany()
+                        .HasForeignKey("EbookId")
+                        .OnDelete(DeleteBehavior.Cascade);
+
+                    b.Navigation("Ebook");
+                });
+
+            modelBuilder.Entity("NexusReader.Domain.Entities.KnowledgeUnitLink", b =>
+                {
+                    b.HasOne("NexusReader.Domain.Entities.KnowledgeUnit", "SourceUnit")
+                        .WithMany("OutgoingLinks")
+                        .HasForeignKey("SourceUnitId")
+                        .OnDelete(DeleteBehavior.Cascade)
+                        .IsRequired();
+
+                    b.HasOne("NexusReader.Domain.Entities.KnowledgeUnit", "TargetUnit")
+                        .WithMany("IncomingLinks")
+                        .HasForeignKey("TargetUnitId")
+                        .OnDelete(DeleteBehavior.Cascade)
+                        .IsRequired();
+
+                    b.Navigation("SourceUnit");
+
+                    b.Navigation("TargetUnit");
+                });
+
+            modelBuilder.Entity("NexusReader.Domain.Entities.NexusUser", b =>
+                {
+                    b.HasOne("NexusReader.Domain.Entities.SubscriptionPlan", "SubscriptionPlan")
+                        .WithMany()
+                        .HasForeignKey("SubscriptionPlanId")
+                        .OnDelete(DeleteBehavior.Restrict)
+                        .IsRequired();
+
+                    b.Navigation("SubscriptionPlan");
+                });
+
+            modelBuilder.Entity("NexusReader.Domain.Entities.QuizResult", b =>
+                {
+                    b.HasOne("NexusReader.Domain.Entities.NexusUser", "User")
+                        .WithMany("QuizResults")
+                        .HasForeignKey("UserId")
+                        .OnDelete(DeleteBehavior.Cascade)
+                        .IsRequired();
+
+                    b.Navigation("User");
+                });
+
+            modelBuilder.Entity("NexusReader.Domain.Entities.Author", b =>
+                {
+                    b.Navigation("Ebooks");
+                });
+
+            modelBuilder.Entity("NexusReader.Domain.Entities.KnowledgeUnit", b =>
+                {
+                    b.Navigation("IncomingLinks");
+
+                    b.Navigation("OutgoingLinks");
+                });
+
+            modelBuilder.Entity("NexusReader.Domain.Entities.NexusUser", b =>
+                {
+                    b.Navigation("Ebooks");
+
+                    b.Navigation("QuizResults");
+                });
+#pragma warning restore 612, 618
+        }
+    }
+}
diff --git a/src/NexusReader.Data/Persistence/Migrations/20260513185108_MakeKnowledgeUnitEbookIdNullable.cs b/src/NexusReader.Data/Persistence/Migrations/20260513185108_MakeKnowledgeUnitEbookIdNullable.cs
new file mode 100644
index 0000000..0236f09
--- /dev/null
+++ b/src/NexusReader.Data/Persistence/Migrations/20260513185108_MakeKnowledgeUnitEbookIdNullable.cs
@@ -0,0 +1,37 @@
+using System;
+using Microsoft.EntityFrameworkCore.Migrations;
+
+#nullable disable
+
+namespace NexusReader.Data.Persistence.Migrations
+{
+    /// <inheritdoc />
+    public partial class MakeKnowledgeUnitEbookIdNullable : Migration
+    {
+        /// <inheritdoc />
+        protected override void Up(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.AlterColumn<Guid>(
+                name: "EbookId",
+                table: "KnowledgeUnits",
+                type: "uuid",
+                nullable: true,
+                oldClrType: typeof(Guid),
+                oldType: "uuid");
+        }
+
+        /// <inheritdoc />
+        protected override void Down(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.AlterColumn<Guid>(
+                name: "EbookId",
+                table: "KnowledgeUnits",
+                type: "uuid",
+                nullable: false,
+                defaultValue: new Guid("00000000-0000-0000-0000-000000000000"),
+                oldClrType: typeof(Guid),
+                oldType: "uuid",
+                oldNullable: true);
+        }
+    }
+}
diff --git a/src/NexusReader.Domain/Entities/KnowledgeUnit.cs b/src/NexusReader.Domain/Entities/KnowledgeUnit.cs
index 5c864f1..56b2bc3 100644
--- a/src/NexusReader.Domain/Entities/KnowledgeUnit.cs
+++ b/src/NexusReader.Domain/Entities/KnowledgeUnit.cs
@@ -11,11 +11,10 @@ public class KnowledgeUnit
     [MaxLength(128)]
     public string Id { get; set; } = string.Empty; // Hash(Source + Content + Version)
 
-    [Required]
-    public Guid EbookId { get; set; }
-
-    [ForeignKey(nameof(EbookId))]
-    public virtual Ebook Ebook { get; set; } = null!;
+    public Guid? EbookId { get; set; }
+ 
+     [ForeignKey(nameof(EbookId))]
+    public virtual Ebook? Ebook { get; set; }
 
     [Required]
     [MaxLength(50)]
diff --git a/src/NexusReader.Infrastructure/Services/KnowledgeService.cs b/src/NexusReader.Infrastructure/Services/KnowledgeService.cs
index 8813758..cf94797 100644
--- a/src/NexusReader.Infrastructure/Services/KnowledgeService.cs
+++ b/src/NexusReader.Infrastructure/Services/KnowledgeService.cs
@@ -209,10 +209,8 @@ public class KnowledgeService : IKnowledgeService
             unit.Content = unitDto.Content;
             
             // Link to the specific ebook if provided. 
-            // If not provided, we should ideally have a "Global" ebook or make it nullable.
-            // For now, we use Guid.Empty which will likely fail FK if not present,
-            // or we make the property nullable in the entity.
-            unit.EbookId = ebookId ?? Guid.Empty; 
+            // Link to ebook if provided
+            unit.EbookId = ebookId; 
             
             unit.MetadataJson = JsonSerializer.Serialize(unitDto.Metadata);