I am using loopback version 2.x.x. It turns out to be very simple

currentModel.app.models.modelYouWantToAccess 

Try with below

var  employee_data = Employee  .filter(x => User.map(y => y.accountId).includes(x.accountId));

In Loopback 4

Solution 1:

In your user controller where you have the access to the UserRepository

Assuming you know the accountId and the accountId only exists if you have an employee record you can have a method

@get('/users/{accountId})
async findUserWhoIsAnEmployee(@param.path.string('accountId') accountId: string){
 return this.userRepository.findOne({where: {accountId}})
}

Assuming you don't know the accountId...

You can access the employee repository in your UserController by passing it as an argument in the constructor function and finding the user, then checking in the Employee record. (Not optimal since two queries will be made on your DB)

export class UserController{
  constructor(
     @repository(UserRepository) public userRepository: UserRepository,
     @repository(EmployeeRepository) private employeeRepository: EmployeeRepository
  )
  // returns employee record or null if none
  @get('/users/{id}/employee-record)
  async findUserWhoIsAnEmployee(@param.path.string('id') id: string){
   const user = await this.userRepository.findById(id);
   return (!user) ? null : this.employeeRepository.findOne({where: {accountId: user.accountId}})
  }
}

Solution 2:

Where you have a relation between the User and Employee Model (hasOne, preferrably)

You can find that employee record on the user

// returns employee record of a user if exists 
@get('/users/{id})
async findUserWhoIsAnEmployee(@param.path.string('id') id: string){
 return this.userRepository.findById(id).employeeRecord()
}
//Where employeeRecord is a reference property on the user Model.