feat(schema) require only if_field in conditional fields

This allows us to auto-fill the then_field with a default and
not require the presence of the if_field when performing
a partial update.

kong/db/schema/init.lua

@@ -495,7 +495,8 @@ Schema.entity_checkers = {
   --                 then_match = { required = true } }
   -- ```
   conditional = {
-    field_sources = { "if_field" },
+    field_sources = { "if_field", "then_field" },
+    required_fields = { ["if_field"] = true },
     fn = function(entity, arg, schema)
       local if_value = get_field(entity, arg.if_field)
       local then_value = get_field(entity, arg.then_field) or null
@@ -909,10 +910,12 @@ local function run_entity_check(self, name, input, arg, full_check)
   end
 
   local all_nil = true
+  local required_fields = checker.required_fields
   for _, fname in ipairs(fields_to_check) do
     local value = get_field(input, fname)
     if value == nil then
-      if not checker.run_with_missing_fields then
+      if not (checker.run_with_missing_fields or
+             (required_fields and not required_fields[fname])) then
         local err = validation_errors.REQUIRED_FOR_ENTITY_CHECK
         set_field(field_errors, fname, err)
         ok = false

spec/01-unit/000-new-dao/01-schema/01-schema_spec.lua

@@ -1430,9 +1430,12 @@ describe("schema", function()
         redis_host = "example.com",
         redis_port = 80
       }))
-      assert.falsy(Test:validate_update({
+      assert.truthy(Test:validate_update({
         policy = "bla",
       }))
+      assert.falsy(Test:validate_update({
+        policy = "redis",
+      }))
     end)
 
     it("supports entity checks on nested fields", function()
@@ -1476,6 +1479,11 @@ describe("schema", function()
         }
       }))
       assert.falsy(Test:validate_update({
+        config = {
+          policy = "redis",
+        }
+      }))
+      assert.truthy(Test:validate_update({
         config = {
           policy = "bla",
         }
@@ -1536,7 +1544,7 @@ describe("schema", function()
           { d = { type = "integer" }, },
           { e = { type = "boolean" }, },
           { f = { type = "string" }, },
-                { g = { type = "record", fields = {} }, },
+          { g = { type = "record", fields = {} }, },
           { h = { type = "map", keys = {}, values = {} }, },
         }
       })