RelationLoader doesn't use transaction

[shayded-exe]

Issue type:

[ ] question
[x] bug report
[ ] feature request
[ ] documentation issue

Database system/driver:

[ ] cordova
[ ] mongodb
[ ] mssql
[ ] mysql / mariadb
[ ] oracle
[x] postgres
[ ] cockroachdb
[ ] sqlite
[ ] sqljs
[ ] react-native
[ ] expo

TypeORM version:

[x] latest
[ ] @next
[ ] 0.x.x (or put your version here)

Steps to reproduce or a small repository showing the problem:

When trying to load a relation from an entity created during a transaction, the relation loader doesn't use the transaction from the provided entity manager.

The following code will return no results if the person entity was created during the current transaction:

let results = await manager
  .createQueryBuilder(Person, 'x')
  .relation(key)
  .of(person)
  .loadMany();

[cainlevy]

This one also got me. I went back to the docs and found the behavior partially described:

You also cannot use classes which use global manager or connection to execute their queries.
https://typeorm.io/#/transactions

This is currently happening in RelationQueryBuilder as it tries to access RelationLoader:

typeorm/src/query-builder/RelationQueryBuilder.ts

Line 164 in 5fc802d

return this.connection.relationLoader.load(this.expressionMap.relationMetadata, of);

[shayded-exe]

I don't see why it couldn't be changed to use transactions. @pleerock ? I'd be willing to work on a PR.