public class GetAllProjectsWithSubProjectViewModel
    {
        public Project Project { get; set; }

        public ICollection<SubProject> SubProjects { get; set; }
    }

public class Project
    {
        // PK
        public int Id { get; set; }

        public string ProjectName { get; set; }

        public Project()
        { }

        public Project(string projectName)
        {
            this.ProjectName = projectName;
        }
    }

public class SubProject
    {
        // PK
        public int Id { get; set; }

        public string SubProjectName { get; set; }

        public SubProject()
        { }

        public SubProject(string subProjectName)
        {
            this.SubProjectName = subProjectName;
        }
    }

public class ProjectSubProject
    {
        // PK
        public int ProjectId { get; set; }
        public Project Project { get; set; }

        // PK
        public int SubProjectId { get; set; }
        public SubProject SubProject { get; set; }


        public ProjectSubProject()
        { }

        public ProjectSubProject(
            Project project,
            SubProject subProject)
        {
            Project = project;
            SubProject = subProject;
        }
    }

// Host/projects/GetAllProjectsWithSubProjects
        [Authorize]
        [HttpGet]
        public async Task<object> GetAllProjectsWithSubProjects()
        {
            var projectsInfo = new List<GetAllProjectsWithSubProjectViewModel>();

            foreach (var project in _context.Projects)
            {
                var subProjects = await _context.ProjectSubProjects
                    .Where(ps => ps.ProjectId == project.Id)
                    .Select(ps => ps.SubProject)
                    .ToListAsync();

                var projectInfo = new GetAllProjectsWithSubProjectViewModel
                {
                    Project = project,
                    SubProjects = subProjects,
                };
                projectsInfo.Add(projectInfo);
            }

            return projectsInfo;
        }