Provide default empty object for empty model 'args' field (hasura#1154)

<!-- The PR description should answer 2 important questions: -->

### What

If a model has arguments, but they are all provided by presets, then
previously we would require users to pass an empty `args: {}` argument
like this:

```graphql
query MyQuery
  ActorsByMovieMany(args: {}) {
    actor_id
    movie_id
    name
  }
}
```

There is no need for this, so this PR loosens this restriction, by
providing a default empty value. This means users can also do the above
query with:

```graphql
query MyQuery
  ActorsByMovieMany {
    actor_id
    movie_id
    name
  }
}
```

Because both versions now work, this is a non-breaking change.

### How

Instead of just looking at number of arguments in schema generation,
consider which have been prefilled and provide a default empty value if
there is nothing a user could pass anyway.

V3_GIT_ORIGIN_REV_ID: cf184e42a114df782e1480a8f19548dda31e5992

v3/changelog.md

@@ -8,7 +8,34 @@
 
 ### Changed
 
-- OpenTelemetry service name set to `v3-engine` to avoid confusion with
+- Making `args` non-compulsory for models where all arguments have presets.
+
+Previously, if a model had arguments specified that were all provided by
+presets, then we would require them to pass an empty `args: {}` argument:
+
+```graphql
+query MyQuery
+  ActorsByMovieMany(args: {}) {
+    actor_id
+    movie_id
+    name
+  }
+}
+```
+
+This change loosens the restriction, so now the following query is valid too:
+
+```graphql
+query MyQuery
+  ActorsByMovieMany {
+    actor_id
+    movie_id
+    name
+  }
+}
+```
+
+- OpenTelemetry service name set to `ddn-engine` to avoid confusion with
   `graphql-engine`.
 
 ## [v2024.09.23]

v3/crates/engine/tests/execute/commands/functions/model_argument_presets_select_many/expected.json

@@ -12,6 +12,18 @@
           "movie_id": 3,
           "name": "Robert De Niro"
         }
+      ],
+      "noargs": [
+        {
+          "actor_id": 4,
+          "movie_id": 3,
+          "name": "Al Pacino"
+        },
+        {
+          "actor_id": 5,
+          "movie_id": 3,
+          "name": "Robert De Niro"
+        }
       ]
     }
   },
@@ -23,12 +35,20 @@
           "movie_id": 4,
           "name": "Morgan Freeman"
         }
+      ],
+      "noargs": [
+        {
+          "actor_id": 6,
+          "movie_id": 4,
+          "name": "Morgan Freeman"
+        }
       ]
     }
   },
   {
     "data": {
-      "ActorsByMovieMany": null
+      "ActorsByMovieMany": null,
+      "noargs": null
     },
     "errors": [
       {
@@ -37,6 +57,13 @@
         "extensions": {
           "details": null
         }
+      },
+      {
+        "message": "internal error: ndc_unexpected: ndc_client error: connector error: connector returned status code 400 Bad Request with message: missing argument movie_id",
+        "path": ["noargs"],
+        "extensions": {
+          "details": null
+        }
       }
     ]
   }

v3/crates/engine/tests/execute/commands/functions/model_argument_presets_select_many/request.gql

@@ -4,4 +4,9 @@ query MyQuery {
     movie_id
     name
   }
+  noargs: ActorsByMovieMany {
+    actor_id
+    movie_id
+    name
+  }
 }

v3/crates/graphql/schema/src/model_arguments.rs

@@ -8,16 +8,16 @@ use crate::{
     GDS,
 };
 use lang_graphql::schema as gql_schema;
+use metadata_resolve::Qualified;
 use open_dds::models::ModelName;
 use std::collections::{BTreeMap, HashMap};
 
-use metadata_resolve::Qualified;
-
 /// Creates the `args` input object within which the model
 /// arguments fields will live.
 pub fn get_model_arguments_input_field(
     builder: &mut gql_schema::Builder<GDS>,
     model: &metadata_resolve::ModelWithPermissions,
+    include_empty_default: bool,
 ) -> Result<gql_schema::InputField<GDS>, crate::Error> {
     model
         .graphql_api
@@ -34,6 +34,14 @@ pub fn get_model_arguments_input_field(
                 type_name: arguments_input_config.type_name.clone(),
             });
 
+            // if there are no possible arguments, provide a default of `{}`
+            // so that `args` can be omitted if the user chooses
+            let default_value = if include_empty_default {
+                Some(lang_graphql::ast::value::ConstValue::Object(vec![]))
+            } else {
+                None
+            };
+
             gql_schema::InputField {
                 name: arguments_input_config.field_name.clone(),
                 description: None,
@@ -43,7 +51,7 @@ pub fn get_model_arguments_input_field(
                 field_type: ast::TypeContainer::named_non_null(
                     arguments_input_config.type_name.clone(),
                 ),
-                default_value: None,
+                default_value,
                 deprecation_status: gql_schema::DeprecationStatus::NotDeprecated,
             }
         })
@@ -138,8 +146,26 @@ pub fn add_model_arguments_field(
     parent_field_name: &ast::Name,
     parent_type: &ast::TypeName,
 ) -> Result<(), crate::Error> {
+    // which arguments are actually available for the user to provide?
+    let user_arguments: Vec<_> = model
+        .model
+        .arguments
+        .keys()
+        .filter(|argument_name| {
+            for permission in model.select_permissions.values() {
+                // if there is a preset for this argument, it will not be included in the schema
+                if permission.argument_presets.contains_key(*argument_name) {
+                    return false;
+                }
+            }
+            true
+        })
+        .collect();
+
     if !model.model.arguments.is_empty() {
-        let model_arguments_input = get_model_arguments_input_field(builder, model)?;
+        let include_empty_default = user_arguments.is_empty();
+        let model_arguments_input =
+            get_model_arguments_input_field(builder, model, include_empty_default)?;
 
         let name = model_arguments_input.name.clone();