Throw better exception for many-to-many with same navigation on both …

…ends (#25990)

Co-authored-by: Andriy Svyryd <AndriySvyryd@users.noreply.github.com>

src/EFCore/Metadata/Builders/CollectionCollectionBuilder.cs

@@ -3,6 +3,7 @@
 
 using System;
 using System.ComponentModel;
+using Microsoft.EntityFrameworkCore.Diagnostics;
 using Microsoft.EntityFrameworkCore.Infrastructure;
 using Microsoft.EntityFrameworkCore.Metadata.Internal;
 using Microsoft.EntityFrameworkCore.Utilities;
@@ -41,6 +42,12 @@ public CollectionCollectionBuilder(
             Check.DebugAssert(((IConventionSkipNavigation)leftNavigation).IsInModel, "Not in model");
             Check.DebugAssert(((IConventionSkipNavigation)rightNavigation).IsInModel, "Not in model");
 
+            if (leftNavigation == rightNavigation)
+            {
+                throw new InvalidOperationException(
+                    CoreStrings.ManyToManyOneNav(leftEntityType.DisplayName(), leftNavigation.Name));
+            }
+
             LeftEntityType = leftEntityType;
             RightEntityType = rightEntityType;
             LeftNavigation = leftNavigation;

src/EFCore/Properties/CoreStrings.resx

@@ -1018,6 +1018,9 @@
     <value>'{contextType}' generated value '{keyValue}' for the property '{3_entityType}.{2_property}'.</value>
     <comment>Debug CoreEventId.ValueGenerated string object? string string</comment>
   </data>
+  <data name="ManyToManyOneNav" xml:space="preserve">
+    <value>The navigation '{entityType}.{navigation}' cannot be used for both sides of a many-to-many relationship. Many-to-many relationships must use two distinct navigation properties.</value>
+  </data>
   <data name="MissingBackingField" xml:space="preserve">
     <value>The specified field '{field}' could not be found for property '{2_entityType}.{1_property}'.</value>
   </data>

test/EFCore.Tests/ModelBuilding/ManyToManyTestBase.cs

@@ -472,12 +472,13 @@ public virtual void Throws_for_self_ref_with_same_navigation()
             {
                 var modelBuilder = CreateModelBuilder();
 
-                modelBuilder.Entity<SelfRefManyToOne>().Ignore(s => s.SelfRef1);
-                modelBuilder.Entity<SelfRefManyToOne>().HasMany(t => t.SelfRef2)
-                    .WithMany(t => t.SelfRef2);
-
-                Assert.Equal(CoreStrings.EntityRequiresKey("SelfRefManyToOneSelfRefManyToOne (Dictionary<string, object>)"),
-                    Assert.Throws<InvalidOperationException>(() => modelBuilder.FinalizeModel()).Message);
+                Assert.Equal(
+                    CoreStrings.ManyToManyOneNav(nameof(SelfRefManyToOne), nameof(SelfRefManyToOne.SelfRef2)),
+                    Assert.Throws<InvalidOperationException>(
+                        () => modelBuilder
+                            .Entity<SelfRefManyToOne>()
+                            .HasMany(e => e.SelfRef2)
+                            .WithMany(e => e.SelfRef2)).Message);
             }
 
             [ConditionalFact]