Add descriptions for LookupArgumentSets (#21)

OpenAPI 3.0 does not support parameter dependencies and mutually exclusive parameters.

https://swagger.io/docs/specification/describing-parameters/#dependencies

This is a complex topic with a lot of debate:
OAI/OpenAPI-Specification#256

We know that our LookupArgumentSets are mutually exclusive, so the best thing is to
add a description explaining this, so that it will appear in documentation and tools like the swagger editor.

Apia already returns a 400 with a friendly error message.

closes: #4

examples/core_api/argument_sets/object_lookup.rb

@@ -8,7 +8,9 @@ class ObjectLookup < Apia::LookupArgumentSet
       description "Provides for objects to be looked up"
 
       argument :id, type: :string
-      argument :permalink, type: :string
+      argument :permalink, type: :string do
+        description "The permalink of the object to look up"
+      end
 
       potential_error "ObjectNotFound" do
         code :object_not_found

lib/apia/open_api/helpers.rb

@@ -38,6 +38,12 @@ def generate_id_from_definition(definition)
         definition.id.gsub(/\//, "_")
       end
 
+      def formatted_description(description)
+        return description if description.end_with?(".")
+
+        "#{description}."
+      end
+
     end
   end
 end

lib/apia/open_api/objects/parameters.rb

@@ -34,17 +34,7 @@ def initialize(spec:, argument:, route_spec:)
 
         def add_to_spec
           if @argument.type.argument_set?
-            # complex argument sets are not supported in query params (e.g. nested objects)
-            @argument.type.klass.definition.arguments.each_value do |child_arg|
-              param = {
-                name: "#{@argument.name}[#{child_arg.name}]",
-                in: "query",
-                schema: {
-                  type: convert_type_to_open_api_data_type(child_arg.type)
-                }
-              }
-              add_to_parameters(param)
-            end
+            generate_argument_set_params
           elsif @argument.array?
             if @argument.type.enum? || @argument.type.object?
               items = generate_schema_ref(@argument.type.klass.definition)
@@ -87,6 +77,27 @@ def add_to_spec
 
         private
 
+        # Complex argument sets are not supported in query params (e.g. nested objects)
+        # For any LookupArgumentSet only one argument is expected to be provided.
+        # However, OpenAPI does not currently support describing mutually exclusive query params.
+        # refer to: https://swagger.io/docs/specification/describing-parameters/#dependencies
+        def generate_argument_set_params
+          @argument.type.klass.definition.arguments.each_value do |child_arg|
+            param = {
+              name: "#{@argument.name}[#{child_arg.name}]",
+              in: "query",
+              schema: {
+                type: convert_type_to_open_api_data_type(child_arg.type)
+              }
+            }
+            description = []
+            description << formatted_description(child_arg.description) if child_arg.description.present?
+            description << "All '#{@argument.name}[]' params are mutually exclusive, only one can be provided."
+            param[:description] = description.join(" ")
+            add_to_parameters(param)
+          end
+        end
+
         def add_to_parameters(param)
           @route_spec[:parameters] << param
         end

lib/apia/open_api/objects/schema.rb

@@ -70,6 +70,7 @@ def build_schema_for_polymorph
         def generate_child_schemas
           if @definition.type.argument_set?
             @children = @definition.type.klass.definition.arguments.values
+            @schema[:description] = "All '#{@definition.name}[]' params are mutually exclusive, only one can be provided."
           elsif @definition.type.object?
             @children = @definition.type.klass.definition.fields.values
           elsif @definition.type.enum?

spec/support/fixtures/openapi.json

@@ -22,14 +22,16 @@
             "in": "query",
             "schema": {
               "type": "string"
-            }
+            },
+            "description": "All 'time[]' params are mutually exclusive, only one can be provided."
           },
           {
             "name": "time[string]",
             "in": "query",
             "schema": {
               "type": "string"
-            }
+            },
+            "description": "All 'time[]' params are mutually exclusive, only one can be provided."
           },
           {
             "name": "timezone",
@@ -384,14 +386,16 @@
             "in": "query",
             "schema": {
               "type": "string"
-            }
+            },
+            "description": "All 'object[]' params are mutually exclusive, only one can be provided."
           },
           {
             "name": "object[permalink]",
             "in": "query",
             "schema": {
               "type": "string"
-            }
+            },
+            "description": "The permalink of the object to look up. All 'object[]' params are mutually exclusive, only one can be provided."
           },
           {
             "name": "scalar",
@@ -520,14 +524,16 @@
             "in": "query",
             "schema": {
               "type": "string"
-            }
+            },
+            "description": "All 'time[]' params are mutually exclusive, only one can be provided."
           },
           {
             "name": "time[string]",
             "in": "query",
             "schema": {
               "type": "string"
-            }
+            },
+            "description": "All 'time[]' params are mutually exclusive, only one can be provided."
           },
           {
             "name": "timezone",
@@ -618,6 +624,7 @@
         ]
       },
       "CoreAPI_ArgumentSets_TimeLookupArgumentSet": {
+        "description": "All 'time[]' params are mutually exclusive, only one can be provided.",
         "type": "object",
         "properties": {
           "unix": {
@@ -742,6 +749,7 @@
         }
       },
       "CoreAPI_ArgumentSets_ObjectLookup": {
+        "description": "All 'object[]' params are mutually exclusive, only one can be provided.",
         "type": "object",
         "properties": {
           "id": {