loadManyToManyRelationIdsAndGroup returns undefined `related` for existing entities

Issue Description

Until yesterday all of our entities had an integer id. Now, some of them do have an UUID instead. So, their id property is of type string and saved in a BIN(16) column. In order to query an UUID entity, we need to transform the UUID string into a Buffer. Sadly TypeORM does not do that on querying relations (having a binary id) but we can work around that by, among other places, using a transformer in the entity.

@RelationId((table1Entity: Table1Entity) => table1Entity.table2Entity)
@Column({ transformer: new UUIDTransformer() })   // <-- Transform string to buffer and from buffer to string
public propertyName;

Since we do use a DataLoader, we use RelationIdLoader#loadManyToManyRelationIdsAndGroup to perform batch loading.

// If we have entityMetadatas, we can create the loaders for the typeorm relations
    if (connection.entityMetadatas) {
      // For every entity, we create an entry in the loaders
      connection.entityMetadatas.forEach(entityMetadata => {
        const resolverName = entityMetadata.targetName;
        if (!resolverName) return;

        // Add the loader for the entity
        if (!loaders[resolverName]) loaders[resolverName] = {};

        // For every relation, we create a dataLoader
        entityMetadata.relations.forEach(relation => {
          if (!loaders[resolverName][relation.propertyName]) {
            // Create the new loader
            loaders[resolverName][relation.propertyName] = new DataLoader((entities: any[]) => {
              return connection.relationIdLoader.loadManyToManyRelationIdsAndGroup(relation, entities).then(groups => groups.map(group => group.related));
            });
          }
        });
      });
    }

Expected Behavior

Using RelationIdLoader#loadManyToManyRelationIdsAndGroup should give us the related entities of the given UUID entities, as it still works for entities with an integer id (not needing a transformer).

Actual Behavior

The related property of RelationIdLoader#loadManyToManyRelationIdsAndGroup `s return value is undefined for every entity, since the transformer is not applied for the ids (see first comment in this issue).

const result = await this.relationIdLoader.loadManyToManyRelationIdsAndGroup(relation, entities);
console.log(result);
/*
[
  {
    entity: { id: <Buffer 10 eb 35 83 dd 7e 6c 50 87 5e 71 2c 19 bb a1 5a> },
    related: undefined,
  },
  ...
]
*/

My Environment

Dependency	Version
Node.js version	v10.20.1
Typescript version	v3.8.3
TypeORM version	v0.2.24
<!--	Operating System	-->

Relevant Database Driver(s)

[x] MariaDB
[ ] aurora-data-api
[ ] aurora-data-api-pg
[ ] better-sqlite3
[ ] cockroachdb
[ ] cordova
[ ] expo
[ ] mongodb
[ ] mysql
[ ] nativescript
[ ] oracle
[ ] postgres
[ ] react-native
[ ] sap
[ ] sqlite
[ ] sqlite-abstract
[ ] sqljs
[ ] sqlserver

Are you willing to resolve this issue by submitting a Pull Request?

[ ] Yes, I have the time, and I know how to start.
[ ] Yes, I have the time, but I don't know how to start. I would need guidance.
[x] No, I don't have the time, although I believe I could do it if I had the time...
[ ] No, I don't have the time and I wouldn't even know how to start.

typeorm / typeorm