From c53eae58cd7bee20d4dac0e0a92e70c514989ada Mon Sep 17 00:00:00 2001
From: Zach Daniel <zach@zachdaniel.dev>
Date: Sun, 2 Oct 2022 20:46:30 -0400
Subject: [PATCH] improvement: support zero argument functions in `compare/2`

---
 lib/ash/resource/validation/builtins.ex |  2 ++
 lib/ash/resource/validation/compare.ex  | 27 ++++++++++++++++---------
 2 files changed, 20 insertions(+), 9 deletions(-)

diff --git a/lib/ash/resource/validation/builtins.ex b/lib/ash/resource/validation/builtins.ex
index bd827031..b3b3cb53 100644
--- a/lib/ash/resource/validation/builtins.ex
+++ b/lib/ash/resource/validation/builtins.ex
@@ -97,6 +97,8 @@ defmodule Ash.Resource.Validation.Builtins do
   @numericality_docs """
   Validates that an attribute or argument meets the given comparison criteria.
 
+  The values provided for each option may be a literal value, attribute, argument, or a zero argument function.
+
   ## Options
   #{Spark.OptionsHelpers.docs(Keyword.delete(Ash.Resource.Validation.Compare.opt_schema(), :attribute))}
 
diff --git a/lib/ash/resource/validation/compare.ex b/lib/ash/resource/validation/compare.ex
index 1894b476..7c54b8cf 100644
--- a/lib/ash/resource/validation/compare.ex
+++ b/lib/ash/resource/validation/compare.ex
@@ -12,22 +12,22 @@ defmodule Ash.Resource.Validation.Compare do
       doc: "The attribute to check"
     ],
     greater_than: [
-      type: {:or, [:integer, :atom]},
+      type: {:or, [:integer, :atom, {:fun, 0}]},
       required: false,
-      doc: "The value that the attribute should be greater than"
+      doc: "The value that the attribute should be greater than."
     ],
     greater_than_or_equal_to: [
-      type: {:or, [:integer, :atom]},
+      type: {:or, [:integer, :atom, {:fun, 0}]},
       required: false,
       doc: "The value that the attribute should be greater than or equal to"
     ],
     less_than: [
-      type: {:or, [:integer, :atom]},
+      type: {:or, [:integer, :atom, {:fun, 0}]},
       required: false,
       doc: "The value that the attribute should be less than"
     ],
     less_than_or_equal_to: [
-      type: {:or, [:integer, :atom]},
+      type: {:or, [:integer, :atom, {:fun, 0}]},
       required: false,
       doc: "The value that the attribute should be less than or equal to"
     ]
@@ -63,7 +63,7 @@ defmodule Ash.Resource.Validation.Compare do
               {:greater_than, attribute} ->
                 if Comp.greater_than?(value, attribute_value(changeset, attribute)),
                   do: :ok,
-                  else: invalid_attribute_error(opts, "must be greater than #{attribute}")
+                  else: invalid_attribute_error(opts, "must be greater than %{value}")
 
               {:greater_than_or_equal_to, attribute} ->
                 if Comp.greater_or_equal?(value, attribute_value(changeset, attribute)),
@@ -71,13 +71,13 @@ defmodule Ash.Resource.Validation.Compare do
                   else:
                     invalid_attribute_error(
                       opts,
-                      "must be greater than or equal to #{attribute}"
+                      "must be greater than or equal to %{value}"
                     )
 
               {:less_than, attribute} ->
                 if Comp.less_than?(value, attribute_value(changeset, attribute)),
                   do: :ok,
-                  else: invalid_attribute_error(opts, "must be less than #{attribute}")
+                  else: invalid_attribute_error(opts, "must be less than %{value}")
 
               {:less_than_or_equal_to, attribute} ->
                 if Comp.less_or_equal?(value, attribute_value(changeset, attribute)),
@@ -85,7 +85,7 @@ defmodule Ash.Resource.Validation.Compare do
                   else:
                     invalid_attribute_error(
                       opts,
-                      "must be less than or equal to #{attribute}"
+                      "must be less than or equal to %{value}"
                     )
 
               true ->
@@ -99,6 +99,10 @@ defmodule Ash.Resource.Validation.Compare do
     end
   end
 
+  defp attribute_value(_changeset, attribute) when is_function(attribute, 0) do
+    attribute.()
+  end
+
   defp attribute_value(changeset, attribute) when is_atom(attribute),
     do: Ash.Changeset.get_argument_or_attribute(changeset, attribute)
 
@@ -110,6 +114,11 @@ defmodule Ash.Resource.Validation.Compare do
        field: opts[:attribute],
        message: opts[:message] || message,
        vars: [
+         value:
+           case opts[:attribute] do
+             fun when is_function(fun, 0) -> fun.()
+             v -> v
+           end,
          greater_than: opts[:greater_than],
          less_than: opts[:less_than],
          greater_than_or_equal_to: opts[:greater_than_or_equal_to],