Conditional Validation in Ecto for OR - 1 of 2 fields is required

How about:

  def validate_required_inclusion(changeset, fields,  options \\ []) do
    if Enum.any?(fields, fn(field) -> get_field(changeset, field) end), 
      do: changeset,
      else: add_error(changeset, hd(fields), "One of these fields must be present: #{inspect fields}")
  end

get_field gives you fields accepted by the change set, both changed (cast) and non-changed, and Enum.any? will ensure that at least one of the field is in there.

You could also create the constraint in the database, e.g. by writing a migration:

create(
  constraint(
    :users,
    :email_or_mobile,
    check: "(email IS NOT NULL) OR (mobile IS NOT NULL)"
  )
)

And use check_constraint to validate the changeset:

def changeset(struct, params \\ %{}) do
  struct
  |> cast(params, [:email, :first_name, :last_name, :password_hash, :role, :birthdate, :address1, :address2, :city, :state, :zip, :status, :mobile, :card, :sms_code, :status])
  |> check_constraint(
    :users_table,
    name: :email_or_mobile,
    message: dgettext("errors", "can't be blank")
  )
end

Here's a simple way. You can customize it to support better error messages:

def validate_required_inclusion(changeset, fields) do
  if Enum.any?(fields, &present?(changeset, &1)) do
    changeset
  else
    # Add the error to the first field only since Ecto requires a field name for each error.
    add_error(changeset, hd(fields), "One of these fields must be present: #{inspect fields}")
  end
end

def present?(changeset, field) do
  value = get_field(changeset, field)
  value && value != ""
end

Test with a Post model and |> validate_required_inclusion([:title , :content]):

iex(1)> Post.changeset(%Post{}, %{})
#Ecto.Changeset<action: nil, changes: %{},
 errors: [title: {"One of these fields must be present: [:title, :content]",
   []}], data: #MyApp.Post<>, valid?: false>
iex(2)> Post.changeset(%Post{}, %{title: ""})
#Ecto.Changeset<action: nil, changes: %{},
 errors: [title: {"One of these fields must be present: [:title, :content]",
   []}], data: #MyApp.Post<>, valid?: false>
iex(3)> Post.changeset(%Post{}, %{title: "foo"})
#Ecto.Changeset<action: nil, changes: %{title: "foo"}, errors: [],
 data: #MyApp.Post<>, valid?: true>
iex(4)> Post.changeset(%Post{}, %{content: ""})
#Ecto.Changeset<action: nil, changes: %{},
 errors: [title: {"One of these fields must be present: [:title, :content]",
   []}], data: #MyApp.Post<>, valid?: false>
iex(5)> Post.changeset(%Post{}, %{content: "foo"})
#Ecto.Changeset<action: nil, changes: %{content: "foo"}, errors: [],
 data: #MyApp.Post<>, valid?: true>