[BUG] [Pistache] Validation is incomplete on model structs which have a child model struct

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Sponsorship to speed up the bug fix or feature request (example)

Description

When an object contains another object, the generated validation method doesn't call the generated validation method of the child object.

openapi-generator version

Release 7.0.1

OpenAPI declaration file content or url

Extract of YAML description of the API :

    card_desc:
      type: object
      properties:
        vip_level:
          type: integer
          minimum: 0
        comments:
          description: Additionnal comments
          type: string
          maxLength: 100
      required:
        - vip_level
    card:
      type: object
      properties:
        id:
          description: Unique id = RFID tag id
          type: string
          minLength: 1
          maxLength: 36
        desc:
          description: Description
          $ref: "#/components/schemas/card_desc"
      required:
        - id
        - desc

Generation Details

Generated code for card_desc:

bool Card_desc::validate(std::stringstream& msg, const std::string& pathPrefix) const
{
    bool success = true;
    const std::string _pathPrefix = pathPrefix.empty() ? "Card_desc" : pathPrefix;

    /* vip_level */ {
        const int32_t& value = vip_level;
        const std::string currentValuePath = _pathPrefix + ".vipLevel";       
        if (value < 0)
        {
            success = false;
            msg << currentValuePath << ": must be greater than or equal to 0;";
        }
    }
             
    if (comments.has_value())
    {
        const std::string& value = comments.value();
        const std::string currentValuePath = _pathPrefix + ".comments"; 
        if (value.length() > 100)
        {
            success = false;
            msg << currentValuePath << ": must be at most 100 characters long;";
        }
    } 
    return success;
}

=> Ok, all fields are checked

Generated code for card:

bool Card::validate(std::stringstream& msg, const std::string& pathPrefix) const
{
    bool success = true;
    const std::string _pathPrefix = pathPrefix.empty() ? "Card" : pathPrefix;

    /* id */ {
        const std::string& value = id;
        const std::string currentValuePath = _pathPrefix + ".id";

        if (value.length() < 1)
        {
            success = false;
            msg << currentValuePath << ": must be at least 1 characters long;";
        }
        if (value.length() > 36)
        {
            success = false;
            msg << currentValuePath << ": must be at most 36 characters long;";
        }
    }
        
    return success;
}

=> Ko, the desc member is not validated!

Suggest a fix

Here is a suggestion which fixes the issue :

diff --git a/modules/openapi-generator/src/main/resources/cpp-pistache-server/model-struct-source.mustache b/modules/openapi-generator/src/main/resources/cpp-pistache-server/model-struct-source.mustache
index 4d7dbfe0a90..666d4599d3f 100644
--- a/modules/openapi-generator/src/main/resources/cpp-pistache-server/model-struct-source.mustache
+++ b/modules/openapi-generator/src/main/resources/cpp-pistache-server/model-struct-source.mustache
@@ -56,18 +56,34 @@ bool {{classname}}::validate(std::stringstream& msg, const std::string& pathPref
         const std::string currentValuePath = _pathPrefix + ".{{nameInCamelCase}}";
         {{> model-validation-body }}
     }
-    {{/isArray}}{{^isArray}}{{#hasValidation}} {{! Only generate validation if necessary }}
+    {{/isArray}}
+    {{^isArray}}
+    {{#hasValidation}} {{! Only generate validation if necessary }}
     {{^required}}if ({{name}}.has_value()){{/required}}
     {{#required}}/* {{name}} */ {{/required}}{
         const {{{dataType}}}& value = {{name}}{{^required}}.value(){{/required}};
         const std::string currentValuePath = _pathPrefix + ".{{nameInCamelCase}}";
         {{> model-validation-body }}
     }
-    {{/hasValidation}}{{/isArray}}{{/vars}}{{/isEnum}}{{#vendorExtensions.x-is-string-enum-container}}{{#anyOf}}{{#-first}}
+    {{/hasValidation}}
+    {{#isModel}}
+    {{^required}}if ({{name}}.has_value()){{/required}}
+    {{#required}}/* {{name}} */ {{/required}}{
+        const {{{dataType}}}& value = {{name}}{{^required}}.value(){{/required}};
+        const std::string currentValuePath = _pathPrefix + ".{{nameInCamelCase}}";
+        if (!value.validate(msg, currentValuePath))
+        {
+            success = false;
+        }
+    }
+    {{/isModel}}
+    {{/isArray}}{{/vars}}
+    {{/isEnum}}{{#vendorExtensions.x-is-string-enum-container}}{{#anyOf}}{{#-first}}
     if (!m_value.validate(msg))
     {
         success = false;
     }{{/-first}}{{/anyOf}}{{/vendorExtensions.x-is-string-enum-container}}
+
     return success;
 }

Which leads to the following code for card :

bool Card::validate(std::stringstream& msg, const std::string& pathPrefix) const
{
    bool success = true;
    const std::string _pathPrefix = pathPrefix.empty() ? "Card" : pathPrefix;

    /* id */ {
        const std::string& value = id;
        const std::string currentValuePath = _pathPrefix + ".id";
       
        if (value.length() < 1)
        {
            success = false;
            msg << currentValuePath << ": must be at least 1 characters long;";
        }
        if (value.length() > 36)
        {
            success = false;
            msg << currentValuePath << ": must be at most 36 characters long;";
        }
    }
                
    /* desc */ {
        const org::openapitools::server::model::Card_desc& value = desc;
        const std::string currentValuePath = _pathPrefix + ".desc";
        if (!value.validate(msg, currentValuePath))
        {
            success = false;
        }
    }
    
    return success;
}

Start with modules/openapi-generator/src/main/resources/cpp-pistache-server/model-struct-source.mustache and generate the C++ Pistache models from the supplied YAML declaration. Check that a parent model's validate method invokes validation for its child model, including the child path prefix; done means nested constraints such as vip_level and comments are reported.