CSHARP-2644: Add a convention for storing an _id declared as string in a POCO as ObjectId in the database.

Author: nurih

src/MongoDB.Bson/Serialization/Conventions/StringIdStoredAsObjectIdConvention.cs

@@ -0,0 +1,44 @@
+using MongoDB.Bson.Serialization.IdGenerators;
+using MongoDB.Bson.Serialization.Serializers;
+
+namespace MongoDB.Bson.Serialization.Conventions
+{
+    /// <summary>
+    /// A convention that sets representation of a string id class member to ObjectId in BSON with a StringObjectIdGenerator.
+    /// </summary>
+    public class StringIdStoredAsObjectIdConvention : ConventionBase, IMemberMapConvention
+    {
+        /// <summary>
+        /// Applies a post processing modification to the class map.
+        /// </summary>
+        /// <param name="memberMap">The BsonMemberMap map.</param>
+        /// <remarks>This method sets both the serializer and the IdGenerator on the id member field.</remarks>
+        public void Apply(BsonMemberMap memberMap)
+        {
+            var idMemberMap = memberMap.ClassMap?.IdMemberMap;
+
+            if (idMemberMap == null)
+            {
+                return;
+            }
+
+            if (idMemberMap.MemberType != typeof(string))
+            {
+                return;
+            }
+
+            if (idMemberMap.IdGenerator != null)
+            {
+                return;
+            }
+
+            var idSerializer = idMemberMap.GetSerializer();
+
+            if (idSerializer is StringSerializer stringSerializer && stringSerializer.Representation == BsonType.String)
+            {
+                idMemberMap.SetSerializer(new StringSerializer(representation: BsonType.ObjectId));
+                idMemberMap.SetIdGenerator(StringObjectIdGenerator.Instance);
+            }
+        }
+    }
+}

tests/MongoDB.Bson.Tests/Serialization/Conventions/StringIdStoredAsObjectIdConventionTests.cs

@@ -0,0 +1,95 @@
+using Xunit;
+using MongoDB.Bson.Serialization.Serializers;
+using MongoDB.Bson.Serialization;
+using MongoDB.Bson.Serialization.IdGenerators;
+using MongoDB.Bson.Serialization.Conventions;
+
+namespace MongoDB.Bson.Tests.Serialization.Conventions
+{
+    public class StringIdStoredAsObjectIdConventionTests
+    {
+        BsonMemberMap SampleMap<T>() => new BsonClassMap<T>(cm => cm.AutoMap()).GetMemberMap("Id");
+
+        [Fact]
+        public void Apply_StringId_SetsSerializer()
+        {
+            var target = new StringIdStoredAsObjectIdConvention();
+            var subject = SampleMap<TestClassWithStringId>();
+
+            target.Apply(subject);
+
+            Assert.IsType<StringSerializer>(subject.GetSerializer());
+        }
+
+        [Fact]
+        public void Apply_StringId_SetsIdGenerator()
+        {
+            var target = new StringIdStoredAsObjectIdConvention();
+            var subject = SampleMap<TestClassWithStringId>();
+
+            target.Apply(subject);
+
+            Assert.IsType<StringObjectIdGenerator>(subject.IdGenerator);
+        }
+
+        [Fact]
+        public void Apply_ExistingIdGenerator_DoesNotApply()
+        {
+            var target = new StringIdStoredAsObjectIdConvention();
+            var subject = SampleMap<TestClassWithStringId>();
+            subject.SetIdGenerator(CombGuidGenerator.Instance);
+
+            target.Apply(subject);
+
+            Assert.IsType<CombGuidGenerator>(subject.IdGenerator);
+        }
+
+        [Fact]
+        public void Apply_NotStringSerializer_DoesNotApply()
+        {
+            var target = new StringIdStoredAsObjectIdConvention();
+            var subject = SampleMap<TestClassWithStringId>();
+            subject.SetSerializer(new FakeStringSerializer());
+
+            target.Apply(subject);
+
+            Assert.IsType<FakeStringSerializer>(subject.GetSerializer());
+        }
+
+
+        [Fact]
+        public void Apply_IntId_LeavesSerializer()
+        {
+            var target = new StringIdStoredAsObjectIdConvention();
+            var subject = SampleMap<TestClassWithIntId>();
+
+            target.Apply(subject);
+
+            Assert.IsNotType<StringSerializer>(subject.GetSerializer());
+        }
+
+        [Fact]
+        public void Apply_IntId_NoIdGenerator()
+        {
+            var target = new StringIdStoredAsObjectIdConvention();
+            var subject = SampleMap<TestClassWithIntId>();
+
+            target.Apply(subject);
+
+            Assert.Null(subject.IdGenerator);
+        }
+
+
+        public class TestClassWithStringId { public string Id; }
+
+        public class TestClassWithIntId { public int Id; }
+
+        class FakeStringSerializer : SealedClassSerializerBase<string>
+        {
+            public BsonType Representation => BsonType.String;
+        }
+    }
+}
+
+
+