[5.0.0-rc2] Throw for ForeignKeyAttribute on skip navigation

src/EFCore/Metadata/Conventions/ForeignKeyAttributeConvention.cs

@@ -302,7 +302,7 @@ private static ForeignKeyAttribute GetForeignKeyAttribute(IConventionTypeBase en
                         && Attribute.IsDefined(p, typeof(ForeignKeyAttribute), inherit: true))
                 ?.GetCustomAttribute<ForeignKeyAttribute>(inherit: true);
 
-        private static ForeignKeyAttribute GetForeignKeyAttribute(IConventionNavigation navigation)
+        private static ForeignKeyAttribute GetForeignKeyAttribute(IConventionNavigationBase navigation)
             => GetAttribute<ForeignKeyAttribute>(navigation.GetIdentifyingMemberInfo());
 
         private static InversePropertyAttribute GetInversePropertyAttribute(IConventionNavigation navigation)
@@ -451,6 +451,17 @@ var fkPropertyOnPrincipal
                         }
                     }
                 }
+
+                foreach (var declaredSkipNavigation in entityType.GetDeclaredSkipNavigations())
+                {
+                    var fkAttribute = GetForeignKeyAttribute(declaredSkipNavigation);
+                    if (fkAttribute != null
+                        && declaredSkipNavigation.ForeignKey?.GetPropertiesConfigurationSource() != ConfigurationSource.Explicit)
+                    {
+                        throw new InvalidOperationException(
+                            CoreStrings.FkAttributeOnSkipNavigation(entityType.DisplayName(), declaredSkipNavigation.Name));
+                    }
+                }
             }
         }
     }

src/EFCore/Properties/CoreStrings.resx

@@ -447,6 +447,9 @@
   <data name="FkAttributeOnPropertyNavigationMismatch" xml:space="preserve">
     <value>The [ForeignKey] attributes on property '{property}' and navigation '{navigation}' in entity type '{entityType}' do not point at each other. The value of the [ForeignKey] attribute on the property should be navigation name, and the value of the [ForeignKey] attribute on the navigation should be the foreign key property name.</value>
   </data>
+  <data name="FkAttributeOnSkipNavigation" xml:space="preserve">
+    <value>The [ForeignKey] attribute cannot be specified on the skip navigation '{entityType}'.'{navigation}'. Configure the foreign key properties in 'OnModelCreating' instead.</value>
+  </data>
   <data name="ForeignKeyCountMismatch" xml:space="preserve">
     <value>The number of properties specified for the foreign key {foreignKeyProperties} on entity type '{dependentType}' does not match the number of properties in the principal key {principalKey} on entity type '{principalType}'.</value>
   </data>

test/EFCore.Tests/ModelBuilding/ManyToManyTestBase.cs

@@ -3,6 +3,7 @@
 
 using System;
 using System.Collections.Generic;
+using System.ComponentModel.DataAnnotations.Schema;
 using System.Linq;
 using Microsoft.EntityFrameworkCore.Diagnostics;
 using Microsoft.EntityFrameworkCore.Metadata;
@@ -344,6 +345,55 @@ public virtual void Throws_for_many_to_many_with_only_one_navigation_configured(
                             .WithMany(d => d.ManyToManyPrincipals)).Message);
             }
 
+            [ConditionalFact]
+            public virtual void Throws_for_ForeignKeyAttribute_on_navigation()
+            {
+                var modelBuilder = CreateModelBuilder();
+
+                modelBuilder.Entity<CategoryWithAttribute>();
+
+                Assert.Equal(
+                    CoreStrings.FkAttributeOnSkipNavigation(
+                        nameof(ProductWithAttribute), nameof(Product.Categories)),
+                    Assert.Throws<InvalidOperationException>(
+                        () => modelBuilder.FinalizeModel()).Message);
+            }
+
+            [ConditionalFact]
+            public virtual void Overrides_ForeignKeyAttribute()
+            {
+                var modelBuilder = CreateModelBuilder();
+
+                modelBuilder.Entity<CategoryWithAttribute>()
+                    .HasMany(e => e.Products)
+                    .WithMany(e => e.Categories)
+                    .UsingEntity<Dictionary<string, object>>(
+                        "ProductCategory",
+                        e => e.HasOne<ProductWithAttribute>().WithMany().HasForeignKey("ProductKey"),
+                        e => e.HasOne<CategoryWithAttribute>().WithMany().HasForeignKey("CategoryKey"));
+
+                var model = modelBuilder.FinalizeModel();
+
+                var category = model.FindEntityType(typeof(CategoryWithAttribute));
+                var productsNavigation = category.GetSkipNavigations().Single();
+                var categoryFk = productsNavigation.ForeignKey;
+                Assert.Equal("CategoryKey", categoryFk.Properties.Single().Name);
+            }
+
+            protected class ProductWithAttribute
+            {
+                public int Id { get; set; }
+
+                [ForeignKey("ProductId")]
+                public virtual ICollection<CategoryWithAttribute> Categories { get; set; }
+            }
+
+            protected class CategoryWithAttribute
+            {
+                public int Id { get; set; }
+                public virtual ICollection<ProductWithAttribute> Products { get; set; }
+            }
+
             [ConditionalFact]
             public virtual void Navigation_properties_can_set_access_mode()
             {