How to add a where clause when using "with" on an Eloquent query in Laravel

You can implement where class both tables. Please check following code and comments.

return \App\Proyecto::with(["projectLeaders" => function($query){
  $query->where() //if condition with inner table.
}])->join('empresas', 'id_empresa', '=', 'empresas.id')
                        ->join('tipo_estado_proyecto', 'tipo_estado_proyecto.id', '=', 'proyectos.id_tipo_estado_proyecto')
  ->where() //if condition with main table column.
                        ->select('empresas.*', 'tipo_estado_proyecto.nombre AS nombreEstadoProyecto', 'proyectos.*');

You can use Closure when accessing relation using with. Check below code for more details:

    return \App\Project::with(["projectLeaders" => function($query){
            $query->where('Lastname', 'Smith') //check lastname
        }])->join('companies', 'company_id', '=', 'companies.id')
                            ->join('project_status', 'project_status.id', '=', 'projects.status_id')
                            ->select('companies.*', 'project_status.name AS statusName', 'projects.*');

You may use the where method on a query builder instance to add where clauses to the query. The most basic call to where requires three arguments. The first argument is the name of the column. The second argument is an operator, which can be any of the database's supported operators. Finally, the third argument is the value to evaluate against the column.

    return \App\Project::with("projectLeaders")->join('companies', 'company_id', '=', 'companies.id')
                            ->join('project_status', 'project_status.id', '=', 'projects.status_id')
                            ->where('lastname','=','Smith')
                            ->select('companies.*', 'project_status.name AS statusName', 'projects.*');

Don't forget to return results with a get();

The query you have written is correct. But after building the query you need to fetch the data from database.

METHOD ONE

So adding get() method to your query:

return App\Project::with('projectLeaders')
                    ->leftJoin('companies', 'company_id', '=', 'companies.id')
                    ->leftJoin('project_status', 'project_status.id', '=', 'projects.status_id')
                    ->select('companies.*', 'project_status.name AS statusName', 'projects.*')
                    ->get();

METHOD TWO (with pagination)

return App\Project::with('projectLeaders')
                    ->leftJoin('companies', 'company_id', '=', 'companies.id')
                    ->leftJoin('project_status', 'project_status.id', '=', 'projects.status_id')
                    ->select('companies.*', 'project_status.name AS statusName', 'projects.*')
                    ->paginate(3);