Showing there is no way to dynamically build an Ecto query where the schemas involved and number of joins are only known at runtime

join_test.exs

defmodule MyApp.JoinTest do
  def add_join(query, qual, join_schema, join_key, _other_schema, other_key) do
    # -------------------------->|  |<-------------------------- #
    Ecto.Query.join(query, qual, [t0], t1 in ^(join_schema), field(t1, ^join_key) == field(t0, ^other_key))
    # -------------------------->|  |<-------------------------- #
  end

  test "add_join" do
    query =
      Ecto.Query.from(Cart)
      |> add_join(:inner, CartItem, :cart_id, Cart, :id)
      |> add_join(:inner, Product, :id, CartItem, :product_id)

    expected = Ecto.Query.from(
      t0 in Cart,
      join: t1 in CartItem,
      on: t1.cart_id == t0.id,
      join: t2 in Product,
      on: t2.id == t1.product_id)

    assert inspect(query) == inspect(expected)
    # 1) test add_join (MyApp.JoinTest)
    #    test/lib/my_app/join_test.exs
    #    Assertion with == failed
    #    code: inspect(query) == inspect(expected)
    #    lhs:  "#Ecto.Query<from c0 in MyApp.JoinTest.Cart, join: c1 in MyApp.JoinTest.CartItem, on: c1.cart_id == c0.id, join: p in MyApp.JoinTest.Product, on: p.id == c0.product_id>"
    #    rhs:  "#Ecto.Query<from c0 in MyApp.JoinTest.Cart, join: c1 in MyApp.JoinTest.CartItem, on: c1.cart_id == c0.id, join: p in MyApp.JoinTest.Product, on: p.id == c1.product_id>"
  end
end

It is possible to make every part of a join clause dynamic, except for the bindings (circled in screenshot). The bindings and the number of bindings must be determined at compile time. This makes it (nearly) impossible to create a library that generates arbitrary queries based on runtime data.