I’d say if Ecto does not already setup a typespec for your schema when it defines the struct, then I’d consider that a bug. This should happen automatically IMO.

Could someone check if the LOC below are flawless, especially:

• the virtual field
• the associations

teacher_id: integer() or should it be teacher: Teacher.t()
user_id: integer() or should it be user: User.t()

# user.ex

@type t :: %__MODULE__{
  id: integer,
  email: String.t(),
  name:  String.t(),
  phone: String.t(),

  credential: Credential.t(),
  teacher_id: integer,
  student_id: integer,

  inserted_at: NaiveDateTime.t(),
  updated_at: NaiveDateTime.t()
  }

schema "users" do
  field(:email, :string)
  field(:name, :string)
  field(:phone, :string)

   has_one(:credential, Credential)
   belongs_to(:teacher, Teacher)
   belongs_to(:student, Student)

   timestamps()
end

# credential.ex

@type t :: %__MODULE__{
 password: String.t(),
 password_hash: String.t(),
 
  user_id: integer,

  inserted_at: NaiveDateTime.t(),
  updated_at: NaiveDateTime.t()
}

schema "credentials" do
  field(:password_hash, :string)
  # Virtual Fields
  field(:password, :string, virtual: true)

  belongs_to(:user, User)

  timestamps()
end

# teacher.ex

@type t :: %__MODULE__{
  id: integer,
  experience: String.t(),
  
  user: User.t(),
  students: [ Student.t() ],
  subjects: [ Subject.t() ],

  inserted_at: NaiveDateTime.t(),
  updated_at: NaiveDateTime.t()
}

schema "teachers" do

  field(:experience, :integer, default: 1)

  has_one(:user, User)
  has_many(:student, Student)
  has_many(:subject, Subject)

  timestamps()
  end

Custom types should be implemented using Ecto.Type behaviour, so maybe it would be possible to bake this into any new custom type?

There is also a relevant issue in the ejpcmac/typed_struct repo.