Implement DbSet.Find

Issue #797

Semantics are essentially the same as EF6:
- If entity with given key is being tracked, then return it without a database query
- Otherwise, query the database and track and return entity if found
- Otherwise, return null

src/Microsoft.EntityFrameworkCore.Specification.Tests/Microsoft.EntityFrameworkCore.Specification.Tests.csproj

@@ -48,6 +48,7 @@
     <Compile Include="DataAnnotationFixtureBase.cs" />
     <Compile Include="DataAnnotationTestBase.cs" />
     <Compile Include="F1FixtureBase.cs" />
+    <Compile Include="FindTestBase.cs" />
     <Compile Include="GearsOfWarQueryFixtureBase.cs" />
     <Compile Include="GearsOfWarQueryTestBase.cs" />
     <Compile Include="GraphUpdatesTestBase.cs" />
@@ -210,4 +211,4 @@
   <Target Name="AfterBuild">
   </Target>
   -->
-</Project>
+</Project>

src/Microsoft.EntityFrameworkCore/ChangeTracking/Internal/CompositePrincipalKeyValueFactory.cs

@@ -36,6 +36,13 @@ public CompositePrincipalKeyValueFactory([NotNull] IKey key)
                 : new CompositeComparer();
         }
 
+        /// <summary>
+        ///     This API supports the Entity Framework Core infrastructure and is not intended to be used 
+        ///     directly from your code. This API may change or be removed in future releases.
+        /// </summary>
+        public virtual object CreateFromKeyValues(object[] keyValues)
+            => keyValues.Any(v => v == null) ? null : keyValues;
+
         /// <summary>
         ///     This API supports the Entity Framework Core infrastructure and is not intended to be used 
         ///     directly from your code. This API may change or be removed in future releases.

src/Microsoft.EntityFrameworkCore/ChangeTracking/Internal/IIdentityMap.cs

@@ -31,6 +31,12 @@ public interface IIdentityMap
         /// </summary>
         bool Contains([NotNull] IForeignKey foreignKey, ValueBuffer valueBuffer);
 
+        /// <summary>
+        ///     This API supports the Entity Framework Core infrastructure and is not intended to be used 
+        ///     directly from your code. This API may change or be removed in future releases.
+        /// </summary>
+        InternalEntityEntry TryGetEntry(object[] keyValues);
+
         /// <summary>
         ///     This API supports the Entity Framework Core infrastructure and is not intended to be used 
         ///     directly from your code. This API may change or be removed in future releases.

src/Microsoft.EntityFrameworkCore/ChangeTracking/Internal/IPrincipalKeyValueFactory.cs

@@ -13,6 +13,12 @@ namespace Microsoft.EntityFrameworkCore.ChangeTracking.Internal
     /// </summary>
     public interface IPrincipalKeyValueFactory<TKey>
     {
+        /// <summary>
+        ///     This API supports the Entity Framework Core infrastructure and is not intended to be used 
+        ///     directly from your code. This API may change or be removed in future releases.
+        /// </summary>
+        object CreateFromKeyValues(object[] keyValues);
+
         /// <summary>
         ///     This API supports the Entity Framework Core infrastructure and is not intended to be used 
         ///     directly from your code. This API may change or be removed in future releases.

src/Microsoft.EntityFrameworkCore/ChangeTracking/Internal/IStateManager.cs

@@ -35,6 +35,12 @@ public interface IStateManager
         /// </summary>
         void BeginTrackingQuery();
 
+        /// <summary>
+        ///     This API supports the Entity Framework Core infrastructure and is not intended to be used 
+        ///     directly from your code. This API may change or be removed in future releases.
+        /// </summary>
+        InternalEntityEntry TryGetEntry(IKey key, object[] keyValues);
+
         /// <summary>
         ///     This API supports the Entity Framework Core infrastructure and is not intended to be used 
         ///     directly from your code. This API may change or be removed in future releases.

src/Microsoft.EntityFrameworkCore/ChangeTracking/Internal/IdentityMap.cs

@@ -77,6 +77,17 @@ public virtual bool Contains(IForeignKey foreignKey, ValueBuffer valueBuffer)
                    && _identityMap.ContainsKey(key);
         }
 
+        /// <summary>
+        ///     This API supports the Entity Framework Core infrastructure and is not intended to be used 
+        ///     directly from your code. This API may change or be removed in future releases.
+        /// </summary>
+        public virtual InternalEntityEntry TryGetEntry(object[] keyValues)
+        {
+            InternalEntityEntry entry;
+            var key = PrincipalKeyValueFactory.CreateFromKeyValues(keyValues);
+            return key != null && _identityMap.TryGetValue((TKey)key, out entry) ? entry : null;
+        }
+
         /// <summary>
         ///     This API supports the Entity Framework Core infrastructure and is not intended to be used 
         ///     directly from your code. This API may change or be removed in future releases.

src/Microsoft.EntityFrameworkCore/ChangeTracking/Internal/SimplePrincipalKeyValueFactory.cs

@@ -31,6 +31,13 @@ public SimplePrincipalKeyValueFactory([NotNull] PropertyAccessors propertyAccess
                 : new NoNullsEqualityComparer();
         }
 
+        /// <summary>
+        ///     This API supports the Entity Framework Core infrastructure and is not intended to be used 
+        ///     directly from your code. This API may change or be removed in future releases.
+        /// </summary>
+        public virtual object CreateFromKeyValues(object[] keyValues)
+            => keyValues[0];
+
         /// <summary>
         ///     This API supports the Entity Framework Core infrastructure and is not intended to be used 
         ///     directly from your code. This API may change or be removed in future releases.