经过一天的学习，我找到的解决方案是在模型之间建立非常简单的关系，并在使用 * with *（）调用关系查询时，在附加到关系查询的过滤器函数中实现所有额外的查询逻辑。
请参见www.example.comhttps://laravel.com/docs/8.x/eloquent-relationships#constraining-eager-loads
我的代码现在工作效率高得多，在dev-env中，在大约10 - 20ms内只对数据库进行4次调用，并以正确的形式构造结果数据，以便将整个集合作为JSON响应返回。

// Controller
public function load(Request $request)
{
  $start = Carbon::parse($request->input('start_date'));
  $end = Carbon::parse($request->input('end_date'));

  $projects = Project::select([
    Project::ID,
    Project::NAME,
    Project::START,
    Project::FINISH
  ])
    ->where(Project::START, '<=', $end)
    ->where(Project::FINISH, '>=', $start)
    ->with([
      // Filter results of the model relationship, passing $start and
      // $end dates down to propagate through to the nested layers
      'project_staff' => $this->filter_project_staff($start, $end)
    ])
    ->orderBy(Project::START)
    ->get();

    return response()->json([
      'projects' => $projects
    ]);
}

private function filter_project_staff($start, $end)
{
  return function ($query) use ($start, $end) {
    $query
      ->select([
        ProjectUser::ID,
        ProjectUser::PROJECT_ID,
        ProjectUser::USER_ID,
        User::NAME,
        ProjectUser::ROLE
      ])
      ->whereIn(ProjectUser::STATUS, [
        ProjectUser::STATUS_RESERVED,
        ProjectUser::STATUS_ASSIGNED,
        ProjectUser::STATUS_ACCEPTED
      ])
      ->join(User::TABLE, User::ID, '=', ProjectUser::USER_ID)
      ->join(Assignment::TABLE, Assignment::PROJECT_USER_ID, '=', ProjectUser::ID)
      ->where(Assignment::START, '<=', $end)
      ->where(Assignment::FINISH, '>=', $start)
      ->distinct(User::NAME)
      ->orderBy(ProjectUser::ROLE, 'desc')
      ->orderBy(User::NAME)
      ->with([
        // Filter results of the nested model relationships
        'assignments' => $this->filter_assignments($start, $end),
        'shifts' => $this->filter_shifts($start, $end)
      ]);
  };
}

private function filter_assignments($start, $end)
{
  return function ($query) use ($start, $end) {
    $query->select([
      Assignment::PROJECT_USER_ID,
      Assignment::START,
      Assignment::FINISH,
      Assignment::TRAVEL_OUT,
      Assignment::TRAVEL_HOME
    ])
      ->where(Assignment::START, '<=', $end)
      ->where(Assignment::FINISH, '>=', $start)
      ->orderBy(Assignment::START);
  };
}

private function filter_shifts($start, $end)
{
  return function ($query) use ($start, $end) {
    $query->select([
      WorkShift::ID,
      WorkShift::USER_ID,
      WorkShift::PROJECT_ID,
      WorkShift::ROLE,
      WorkShift::DATE,
      WorkShift::START,
      WorkShift::HOURS,
      WorkShift::STATUS,
      WorkShift::REMARKS
    ])
      ->where(WorkShift::DATE, '>=', $start)
      ->where(WorkShift::DATE, '<=', $end)
      ->orderBy(WorkShift::DATE);
  };
}

///////////////
//Project Model
public function project_staff(): HasMany
{
    return $this->hasMany(ProjectUser::class);
}

////////////////////
// ProjectUser Model
public function project(): BelongsTo
{
    return $this->belongsTo(Project::class);
}

public function assignments(): HasMany
{
    return $this->hasMany(Assignment::class, 'projectUser_id');
}

public function shifts()
{
    // Using awobaz/compoships to simplify creating the relationship using a composite key
    return $this->hasMany(WorkShift::class, ['project_id','user_id'], ['project_id','user_id']);
}

///////////////////
// Assignment Model
public function project_user(): BelongsTo
{
    return $this->belongsTo(ProjectUser::class, 'projectUser_id');
}

// WorkShift Model
public function project_user()
{
    return $this->belongsTo(ProjectUser::class, ['project_id','user_id'], ['project_id','user_id']);
}

非常感谢任何花时间和思想来帮助我解决这个问题的人。非常感谢你的努力。

如前所述，您可以使用Laravel内置的关系，或者，您可以构造一个巨大的查询并将所有数据连接在一起;
就像这样：

<?php
    
$projectUsers = ProjectUser::select([
        'project_users.id', // define aliases for all columns
        'project_users.user_id',
        'users.name',
        'project_users.role',
        'projects.id',
        'projects.name',
        'projects.start',
        'projects.finish',
        'assignments.start',
        'assignments.finish',
        'assignments.travel_out',
        'assignments.travel_home',
        'work_shifts.id',
        'work_shifts.role',
        'work_shifts.date',
        'work_shifts.start',
        'work_shifts.hours',
        'work_shifts.status',
        'work_shifts.remarks'
    ])
    ->join('users', 'users.id', 'project_users.user_id'
    ->leftJoin('assignments', 'assignment.project_user_id', 'project_users.id')
    ->leftJoin('projects', fn ($query) => $query
        ->where('projects.start', '<=', Carbon::now()->endOfMonth())
        ->where('projects.finish', '>=', Carbon::now()->startOfMonth())
    )
    ->leftJoin('work_shifts', fn ($query) => $query
        ->whereColumn('work_shifts.id', 'project_users.id')
        ->whereColumn('work_shifts.project_id', 'projects.id')
    )
    ->whereColumn('project_users.project_id', 'projects.id')
    ->where('assignments.start', '<=', Carbon::now()->endOfMonth())
    ->where('assignments.finish', '>=', Carbon::now()->startOfMonth())
    ->distinct('users.name')
    ->cursor();

foreach ($projectUsers as $user) {
    // ...
}