Narrow compile_env to leaf keys in :persistence and :cache

CHANGELOG.md

@@ -3,6 +3,7 @@
 ## v1.14.0 (unreleased)
 
 * Drop support for Erlang/OTP 25, and Erlang/OTP >= 26 is now required. Dropping support for older versions of Erlang/OTP simply means that this package is not tested with them in CI, and that no compatibility issues are considered bugs.
+* Fix: runtime overrides of `:repo`, `:adapter` (in `:persistence`) and `:ttl` (in `:cache`) via `runtime.exs` no longer cause Elixir's release-boot `validate_compile_env` check to abort. Only the keys that are genuinely baked into compiled code (`:ecto_table_name`, `:ecto_primary_key_type`, and `:cache` → `:enabled`) are now tracked at compile time. No changes required in host applications.
 
 ## v1.13.0
 

lib/fun_with_flags/config.ex

@@ -65,45 +65,51 @@ defmodule FunWithFlags.Config do
   # Used to determine the store module at compile time, which is stored in a
   # module attribute. `Application.compile_env` cannot be used in functions,
   # so here we are.
-  @compile_time_cache_config Application.compile_env(:fun_with_flags, :cache, [])
+  @compile_time_cache_enabled Application.compile_env(
+    :fun_with_flags,
+    [:cache, :enabled],
+    Keyword.fetch!(@default_cache_config, :enabled)
+  )
 
   # If we're not using the cache, then don't bother with
   # the 2-level logic in the default Store module.
   #
   def store_module_determined_at_compile_time do
-    cache_conf = Keyword.merge(
-      @default_cache_config,
-      @compile_time_cache_config
-    )
-
-    if Keyword.get(cache_conf, :enabled) do
+    if @compile_time_cache_enabled do
       FunWithFlags.Store
     else
       FunWithFlags.SimpleStore
     end
   end
 
 
-  # Used to determine the Ecto table name at compile time.
-  @compile_time_persistence_config Application.compile_env(:fun_with_flags, :persistence, [])
-
-
-  def ecto_table_name_determined_at_compile_time do
-    pers_conf = Keyword.merge(
-      @default_persistence_config,
-      @compile_time_persistence_config
-    )
-    Keyword.get(pers_conf, :ecto_table_name)
-  end
-
-
-  def ecto_primary_key_type_determined_at_compile_time do
-    pers_conf = Keyword.merge(
-      @default_persistence_config,
-      @compile_time_persistence_config
-    )
-    Keyword.get(pers_conf, :ecto_primary_key_type)
-  end
+  # Only the specific keys that are read at compile time are tracked here.
+  # `:ecto_table_name` is interpolated into the `schema(...)` macro call in
+  # `FunWithFlags.Store.Persistent.Ecto.Record`, and `:ecto_primary_key_type`
+  # is used in the `@primary_key` module attribute in the same module — both
+  # must be compile-time constants.
+  #
+  # Other keys in the `:persistence` config (`:repo`, `:adapter`) are read at
+  # runtime via `persistence_config/0` and must NOT be captured here. Snapshotting
+  # the whole keyword list would cause Elixir's release-boot `validate_compile_env`
+  # check to abort whenever a host application legitimately overrides a runtime-only
+  # key — for example pointing `:repo` at a read replica in a specific app profile.
+  @compile_time_ecto_table_name Application.compile_env(
+    :fun_with_flags,
+    [:persistence, :ecto_table_name],
+    Keyword.fetch!(@default_persistence_config, :ecto_table_name)
+  )
+
+  @compile_time_ecto_primary_key_type Application.compile_env(
+    :fun_with_flags,
+    [:persistence, :ecto_primary_key_type],
+    Keyword.fetch!(@default_persistence_config, :ecto_primary_key_type)
+  )
+
+
+  def ecto_table_name_determined_at_compile_time, do: @compile_time_ecto_table_name
+
+  def ecto_primary_key_type_determined_at_compile_time, do: @compile_time_ecto_primary_key_type
 
 
   defp persistence_config do