Fields of composite types are always nullable (#565)

### What

We used to mark fields of composite type as not-nullable in the NDC
schema.

This is wrong. Nullability is a property of columns of tables, not of
fields of record types.

This is also demonstrated by the following transcript:
```
postgres=# create table a_table (nullable_text text, not_nullable_text text not null);
CREATE TABLE
postgres=# create table derived_table (a_table a_table, other text);
CREATE TABLE

postgres=# insert into derived_table values (('nullable', 'not nullable'), 'other text');
INSERT 0 1
postgres=# insert into derived_table values ((null, 'not nullable'), 'other text');
INSERT 0 1

postgres=# select (a_table).* from derived_table;
 nullable_text | not_nullable_text
---------------+-------------------
 nullable      | not nullable
               | not nullable
(2 rows)

postgres=# insert into a_table select (a_table).* from derived_table;
INSERT 0 2
postgres=# select * from a_table;
 nullable_text | not_nullable_text
---------------+-------------------
 nullable      | not nullable
               | not nullable
(2 rows)

-- We can easily construct a record with (not_nullable_text=null) when on **the composite type** a_table:
postgres=# insert into derived_table values (('nullable', null), 'other text');
INSERT 0 1
postgres=# select * from derived_table;
          a_table          |   other
---------------------------+------------
 (nullable,"not nullable") | other text
 (,"not nullable")         | other text
 (nullable,)               | other text
(3 rows)

postgres=# select (a_table).* from derived_table;
 nullable_text | not_nullable_text
---------------+-------------------
 nullable      | not nullable
               | not nullable
 nullable      |
(3 rows)

-- ... But we cannot insert this into **the table** a_table.
postgres=# insert into a_table select (a_table).* from derived_table;
ERROR:  null value in column "not_nullable_text" of relation "a_table" violates not-null constraint
DETAIL:  Failing row contains (nullable, null).
```

### How

We simply make the schema endpoint always return nullable fields of
composite types.

---------

Co-authored-by: Samir Talwar <[email protected]>

Author: plcplc

changelog.md

@@ -22,6 +22,8 @@
 
 ### Fixed
 
+- Make fields of composite types nullable in the schema.
+  [#565](https://github.com/hasura/ndc-postgres/pull/565)
 - Allow Native Operations that end with a semicolon when it's easy to remove them.
   [#566](https://github.com/hasura/ndc-postgres/pull/566)
 - Fix nested field relationships.

crates/connectors/ndc-postgres/src/schema/mod.rs

@@ -283,7 +283,9 @@ pub fn get_schema(
                             field_name.clone(),
                             models::ObjectField {
                                 description: field_info.description.clone(),
-                                r#type: type_to_type(&field_info.r#type),
+                                r#type: models::Type::Nullable {
+                                    underlying_type: Box::new(type_to_type(&field_info.r#type)),
+                                },
                                 arguments: BTreeMap::new(),
                             },
                         )

crates/tests/databases-tests/src/citus/snapshots/databases_tests__citus__schema_tests__schema_test__get_schema.snap

@@ -2965,14 +2965,20 @@ expression: result
       "fields": {
         "name": {
           "type": {
-            "type": "named",
-            "name": "text"
+            "type": "nullable",
+            "underlying_type": {
+              "type": "named",
+              "name": "text"
+            }
           }
         },
         "popularity": {
           "type": {
-            "type": "named",
-            "name": "int8"
+            "type": "nullable",
+            "underlying_type": {
+              "type": "named",
+              "name": "int8"
+            }
           }
         }
       }
@@ -2981,17 +2987,23 @@ expression: result
       "fields": {
         "members": {
           "type": {
-            "type": "array",
-            "element_type": {
-              "type": "named",
-              "name": "chara"
+            "type": "nullable",
+            "underlying_type": {
+              "type": "array",
+              "element_type": {
+                "type": "named",
+                "name": "chara"
+              }
             }
           }
         },
         "name": {
           "type": {
-            "type": "named",
-            "name": "text"
+            "type": "nullable",
+            "underlying_type": {
+              "type": "named",
+              "name": "text"
+            }
           }
         }
       }
@@ -3000,17 +3012,23 @@ expression: result
       "fields": {
         "members": {
           "type": {
-            "type": "array",
-            "element_type": {
-              "type": "named",
-              "name": "person_name"
+            "type": "nullable",
+            "underlying_type": {
+              "type": "array",
+              "element_type": {
+                "type": "named",
+                "name": "person_name"
+              }
             }
           }
         },
         "name": {
           "type": {
-            "type": "named",
-            "name": "text"
+            "type": "nullable",
+            "underlying_type": {
+              "type": "named",
+              "name": "text"
+            }
           }
         }
       }
@@ -3093,8 +3111,11 @@ expression: result
       "fields": {
         "only_occurring_here1": {
           "type": {
-            "type": "named",
-            "name": "int8"
+            "type": "nullable",
+            "underlying_type": {
+              "type": "named",
+              "name": "int8"
+            }
           }
         }
       }
@@ -3268,17 +3289,23 @@ expression: result
       "fields": {
         "committees": {
           "type": {
-            "type": "array",
-            "element_type": {
-              "type": "named",
-              "name": "committee"
+            "type": "nullable",
+            "underlying_type": {
+              "type": "array",
+              "element_type": {
+                "type": "named",
+                "name": "committee"
+              }
             }
           }
         },
         "name": {
           "type": {
-            "type": "named",
-            "name": "text"
+            "type": "nullable",
+            "underlying_type": {
+              "type": "named",
+              "name": "text"
+            }
           }
         }
       }
@@ -3301,14 +3328,20 @@ expression: result
       "fields": {
         "address": {
           "type": {
-            "type": "named",
-            "name": "person_address"
+            "type": "nullable",
+            "underlying_type": {
+              "type": "named",
+              "name": "person_address"
+            }
           }
         },
         "name": {
           "type": {
-            "type": "named",
-            "name": "person_name"
+            "type": "nullable",
+            "underlying_type": {
+              "type": "named",
+              "name": "person_name"
+            }
           }
         }
       }
@@ -3319,15 +3352,21 @@ expression: result
         "address_line_1": {
           "description": "Address line No 1",
           "type": {
-            "type": "named",
-            "name": "text"
+            "type": "nullable",
+            "underlying_type": {
+              "type": "named",
+              "name": "text"
+            }
           }
         },
         "address_line_2": {
           "description": "Address line No 2",
           "type": {
-            "type": "named",
-            "name": "text"
+            "type": "nullable",
+            "underlying_type": {
+              "type": "named",
+              "name": "text"
+            }
           }
         }
       }
@@ -3338,15 +3377,21 @@ expression: result
         "first_name": {
           "description": "The first name of a person",
           "type": {
-            "type": "named",
-            "name": "text"
+            "type": "nullable",
+            "underlying_type": {
+              "type": "named",
+              "name": "text"
+            }
           }
         },
         "last_name": {
           "description": "The last name of a person",
           "type": {
-            "type": "named",
-            "name": "text"
+            "type": "nullable",
+            "underlying_type": {
+              "type": "named",
+              "name": "text"
+            }
           }
         }
       }