litedb-org/LiteDB · error · LiteException

0

0

Error message

There is no _id field mapped in your type: {member.DataType.FullName}

What it means

Thrown during DbRef serialization (BsonMapper.cs, the RegisterDbRef serialize closure) when the referenced entity type has no _id member mapped. When serializing a DbRef, LiteDB stores only the foreign key ($id) and collection ($ref); it obtains the id by reading the referenced type's Id member. If none is mapped (e.g. the type is an interface, or has no Id/Id-like property and no [BsonId]), this fires with code 0. This is issue #768.

Source

Thrown at LiteDB/Client/Mapper/BsonMapper.cs:259

        /// <summary>
        /// Register a property as a DbRef - implement a custom Serialize/Deserialize actions to convert entity to $id, $ref only
        /// </summary>
        private static void RegisterDbRefItem(BsonMapper mapper, MemberMapper member, ITypeNameBinder typeNameBinder)
        {
            // get entity
            var entity = mapper.GetEntityMapper(member.DataType);
            
            member.Serialize = (obj, m) =>
            {
                // supports null values when "SerializeNullValues = true"
                if (obj == null) return BsonValue.Null;
                entity.WaitForInitialization();
                
                var idField = entity.Id;

                // #768 if using DbRef with interface with no ID mapped
                if (idField == null) throw new LiteException(0, "There is no _id field mapped in your type: " + member.DataType.FullName);

                var id = idField.Getter(obj);

                var bsonDocument = new BsonDocument
                {
                    ["$id"] = m.Serialize(id.GetType(), id, 0),
                    ["$ref"] = member.DbRefCollectionName
                };

                if (member.DataType != obj.GetType())
                {
                    bsonDocument["$type"] = mapper.SerializeTypeName(obj.GetType());
                }

                return bsonDocument;
            };

            member.Deserialize = (bson, m) =>

View on GitHub (pinned to f906a5f850)

Solutions

Declare the DbRef property as a concrete class that has an Id member (or Id-named property).
Add [BsonId] to the property that should serve as the key on the referenced type.
Use EntityBuilder.DbRef and EntityBuilder.Id to explicitly map the Id on the referenced type.

Example fix

// before
public class Post
{
    public int Id { get; set; }
    [BsonRef] public IAuthor Author { get; set; } // interface, no Id
}

// after
public class Post
{
    public int Id { get; set; }
    [BsonRef] public Author Author { get; set; }
}
public class Author
{
    public int Id { get; set; }
    public string Name { get; set; }
}

Defensive patterns

Strategy: validation

Validate before calling

// Ensure the referenced type has an Id member before relying on DbRef.
var refMapper = BsonMapper.Global.GetEntityMapper(typeof(TRef));
if (refMapper.Id == null)
    throw new InvalidOperationException($"{typeof(TRef)} has no _id mapped; cannot use as DbRef target.");

Try / catch

try { db.GetCollection<T>().Insert(entity); }
catch (LiteException ex) when (ex.Message.Contains("no _id field mapped"))
{
    // add an Id property/[BsonId] to the referenced type and retry
}

Prevention

Declare DbRef properties as concrete classes with an Id member.
Add [BsonId] explicitly when the Id convention does not match.
Avoid interfaces/abstract bases as DbRef property types unless mapped.

When it happens

Trigger: Declaring a [BsonRef] property whose declared type is an interface or abstract base with no Id member, then Inserting/Updating a document whose DbRef serializer runs. Also triggered when the referenced type's Id property was ignored or not mapped.

Common situations: Modeling a reference as an interface (e.g. public IAuthor Author) without a concrete mapped type, removing the Id property from a referenced entity, or applying [BsonIgnore] to the Id of a DbRef target.

Related errors

AI-assisted analysis of litedb-org/LiteDB@f906a5f850 (2026-08-13). Data as JSON: /api/errors/0de1a7be18f31fe2. Report an issue: GitHub.