Add support for additional use cases of free-form objects

Issue OpenAPITools#5969 and PR OpenAPITools#5970 document an issue with rust-server where free form
models are not generated. OpenAPITools#7373 adds some free form model generation to
python-experimental (later ported to other languages, it seems), but this
feature only covers two of three possible use cases. The third use case remains
unimplemented, though it is used in some larger OpenAPI document sets, such as
DMTF's Redfish schema (which contains 33 of these free form objects without
validation).

related functions), but there does not seem to be an adequate replacement for
detecting free form objects in the OpenAPI data model from template context.
Unfortunately, free form objects require special consideration in statically
type-checked languages such as Rust.

quickly closed, presumably because `isFreeFormObject` was still present in the
data model at the time the issue was opened.

This commit adds a global property (default `false`) called
`generateFreeFormModels` (I'm taking suggestions for a better name) which
enables generation for the third use case not addressed by OpenAPITools#7373. Additionally,
isFreeFormObject is added back into the object model for any case where a
free-form object is generated, allowing generators for languages such as Rust
to handle them gracefully.

Author: AmateurECE

modules/openapi-generator/src/main/java/org/openapitools/codegen/CodegenConstants.java

@@ -33,6 +33,7 @@ public class CodegenConstants {
 
     public static final String WITH_XML = "withXml";
     public static final String SKIP_FORM_MODEL = "skipFormModel";
+    public static final String GENERATE_FREE_FORM_MODELS = "generateUnaliasedFreeFormModels";
     /* /end System Properties */
 
     public static final String API_NAME = "apiName";

modules/openapi-generator/src/main/java/org/openapitools/codegen/CodegenModel.java

@@ -66,7 +66,7 @@ public class CodegenModel implements IJsonSchemaValidationProperties {
     public String defaultValue;
     public String arrayModelType;
     public boolean isAlias; // Is this effectively an alias of another simple type
-    public boolean isString, isInteger, isLong, isNumber, isNumeric, isFloat, isDouble, isDate, isDateTime, isDecimal, isShort, isUnboundedInteger, isPrimitiveType, isBoolean;
+    public boolean isString, isInteger, isLong, isNumber, isNumeric, isFloat, isDouble, isDate, isDateTime, isDecimal, isShort, isUnboundedInteger, isPrimitiveType, isBoolean, isFreeFormObject;
     private boolean additionalPropertiesIsAnyType;
     public List<CodegenProperty> vars = new ArrayList<>(); // all properties (without parent's properties)
     public List<CodegenProperty> allVars = new ArrayList<>(); // all properties (with parent's properties)
@@ -949,6 +949,10 @@ public void setIsAnyType(boolean isAnyType) {
         this.isAnyType = isAnyType;
     }
 
+    public boolean getIsFreeFormObject() { return isFreeFormObject; }
+
+    public void setIsFreeFormObject(boolean isFreeFormObject) { this.isFreeFormObject = isFreeFormObject; }
+
     public boolean getIsUuid() { return isUuid; }
 
     public void setIsUuid(boolean isUuid) { this.isUuid = isUuid; }
@@ -1096,7 +1100,7 @@ hasChildren, isMap, isDeprecated, hasOnlyReadOnly, getExternalDocumentation(), g
                 getMinItems(), getMaxLength(), getMinLength(), getExclusiveMinimum(), getExclusiveMaximum(), getMinimum(),
                 getMaximum(), getPattern(), getMultipleOf(), getItems(), getAdditionalProperties(), getIsModel(),
                 getAdditionalPropertiesIsAnyType(), hasDiscriminatorWithNonEmptyMapping,
-                isAnyType, getComposedSchemas(), hasMultipleTypes, isDecimal, isUuid, requiredVarsMap, ref,
+                isAnyType, isFreeFormObject, getComposedSchemas(), hasMultipleTypes, isDecimal, isUuid, requiredVarsMap, ref,
                 uniqueItemsBoolean, schemaIsFromAdditionalProperties, isBooleanSchemaTrue, isBooleanSchemaFalse,
                 format, dependentRequired, contains);
     }

modules/openapi-generator/src/main/java/org/openapitools/codegen/DefaultCodegen.java

@@ -2788,6 +2788,7 @@ protected void updateModelForObject(CodegenModel m, Schema schema) {
             // non-composed object type with no properties + additionalProperties
             // additionalProperties must be null, ObjectSchema, or empty Schema
             addAdditionPropertiesToCodeGenModel(m, schema);
+            m.setIsFreeFormObject(true);
         }
         // process 'additionalProperties'
         setAddProps(schema, m);
@@ -7002,6 +7003,7 @@ protected void updateRequestBodyForMap(CodegenParameter codegenParameter, Schema
             codegenParameter.isContainer = Boolean.TRUE;
             codegenParameter.isMap = Boolean.TRUE;
             codegenParameter.isNullable = codegenProperty.isNullable;
+            codegenParameter.isFreeFormObject = codegenProperty.isFreeFormObject;
 
             // set nullable
             setParameterNullable(codegenParameter, codegenProperty);

modules/openapi-generator/src/main/java/org/openapitools/codegen/DefaultGenerator.java

@@ -489,13 +489,18 @@ void generateModels(List<File> files, List<ModelMap> allModels, List<String> unu
                     // 2. free form with validation
                     // 3. free form that is allOf included in any composed schemas
                     //      this use case arises when using interface schemas
-                    // generators may choose to make models for use case 2 + 3
+                    // Use cases 2 + 3 are enabled by default, but use case 1 is hidden behind global property
+                    // generateFreeFormModels.
                     Schema refSchema = new Schema();
                     refSchema.set$ref("#/components/schemas/" + name);
                     Schema unaliasedSchema = config.unaliasSchema(refSchema);
-                    if (unaliasedSchema.get$ref() == null) {
-                        LOGGER.info("Model {} not generated since it's a free-form object", name);
-                        continue;
+
+                    Boolean generateFreeFormModels = GlobalSettings.getProperty(CodegenConstants.GENERATE_FREE_FORM_MODELS) != null ?
+                            Boolean.valueOf(GlobalSettings.getProperty(CodegenConstants.GENERATE_FREE_FORM_MODELS)) :
+                            getGeneratorPropertyDefaultSwitch(CodegenConstants.GENERATE_FREE_FORM_MODELS, false);
+                    if (unaliasedSchema.get$ref() == null && !generateFreeFormModels) {
+                         LOGGER.info("Model {} not generated since it's a free-form object. To enable this, set generateFreeFormModels=true", name);
+                         continue;
                     }
                 } else if (ModelUtils.isMapSchema(schema)) { // check to see if it's a "map" model
                     // A composed schema (allOf, oneOf, anyOf) is considered a Map schema if the additionalproperties attribute is set