feat: Add WithNullableFieldTransformer to transformers.TransformWithStruct options (#2084)

<!--
Explain what problem this PR addresses
-->

Fixes cloudquery/cloudquery#20286

Add `transformers.WithNullableFieldTransformer(func ...)` to allow the SDK user to have custom rules to decide for a field nullability.
This is especially useful when the user defines its struct with some pointers and some regular value types: we can have the that set `nil` for pointers, non `nil` for non-pointers.

---

Author: jeromewir

transformers/nullable_field.go

@@ -0,0 +1,11 @@
+package transformers
+
+import "reflect"
+
+type NullableFieldTransformer func(field reflect.StructField) bool
+
+func DefaultNullableFieldTransformer(_ reflect.StructField) bool {
+	return true
+}
+
+var _ NullableFieldTransformer = DefaultNullableFieldTransformer

transformers/options.go

@@ -63,6 +63,14 @@ func WithIgnoreInTestsTransformer(transformer IgnoreInTestsTransformer) StructTr
 	}
 }
 
+// WithNullableFieldTransformer overrides how column NotNull will be determined.
+// DefaultNullableFieldTransformer is used as the default.
+func WithNullableFieldTransformer(transformer NullableFieldTransformer) StructTransformerOption {
+	return func(t *structTransformer) {
+		t.nullableFieldTransformer = transformer
+	}
+}
+
 // WithPrimaryKeys allows to specify what struct fields should be used as primary keys
 func WithPrimaryKeys(fields ...string) StructTransformerOption {
 	return func(t *structTransformer) {

transformers/struct.go

@@ -23,6 +23,7 @@ type structTransformer struct {
 	typeTransformer               TypeTransformer
 	resolverTransformer           ResolverTransformer
 	ignoreInTestsTransformer      IgnoreInTestsTransformer
+	nullableFieldTransformer      NullableFieldTransformer
 	unwrapAllEmbeddedStructFields bool
 	structFieldsToUnwrap          []string
 	pkFields                      []string
@@ -161,6 +162,7 @@ func (t *structTransformer) addColumnFromField(field reflect.StructField, parent
 		Type:          columnType,
 		Resolver:      resolver,
 		IgnoreInTests: t.ignoreInTestsTransformer(field),
+		NotNull:       !t.nullableFieldTransformer(field),
 	}
 
 	// Enrich JSON column with detailed schema
@@ -199,6 +201,7 @@ func TransformWithStruct(st any, opts ...StructTransformerOption) schema.Transfo
 		typeTransformer:           DefaultTypeTransformer,
 		resolverTransformer:       DefaultResolverTransformer,
 		ignoreInTestsTransformer:  DefaultIgnoreInTestsTransformer,
+		nullableFieldTransformer:  DefaultNullableFieldTransformer,
 		jsonSchemaNameTransformer: DefaultJSONColumnSchemaNameTransformer,
 		maxJSONTypeSchemaDepth:    DefaultMaxJSONTypeSchemaDepth,
 	}

transformers/struct_test.go

@@ -82,6 +82,11 @@ type (
 		IntCol     int `json:"int_col"`
 		Properties any
 	}
+
+	testNullableField struct {
+		RegularString  string  `json:"regular_string"`
+		NullableString *string `json:"nullable_string"`
+	}
 )
 
 var (
@@ -301,6 +306,22 @@ var (
 			},
 		},
 	}
+
+	expectedTestTableStructNonNullableFields = schema.Table{
+		Name: "test_struct_with_non_nullable_fields",
+		Columns: schema.ColumnList{
+			{
+				Name:    "regular_string",
+				Type:    arrow.BinaryTypes.String,
+				NotNull: true,
+			},
+			{
+				Name:    "nullable_string",
+				Type:    arrow.BinaryTypes.String,
+				NotNull: false,
+			},
+		},
+	}
 )
 
 func TestTableFromGoStruct(t *testing.T) {
@@ -443,6 +464,18 @@ func TestTableFromGoStruct(t *testing.T) {
 			},
 			want: expectedTestTableStructWithCustomAny,
 		},
+		{
+			name: "Should be able to override any nullability for a field",
+			args: args{
+				testStruct: testNullableField{},
+				options: []StructTransformerOption{
+					WithNullableFieldTransformer(func(f reflect.StructField) bool {
+						return f.Type.Kind() == reflect.Pointer
+					}),
+				},
+			},
+			want: expectedTestTableStructNonNullableFields,
+		},
 	}
 
 	for _, tt := range tests {