fix: Implement nested mode for environment variable override

Cargo.toml

@@ -2,12 +2,12 @@
 name = "gonfig"
 version = "0.1.7"
 edition = "2021"
-authors = ["Vasanthkumar Kalaiselvan<itsparser@gmail.com>"]
+authors = ["Vasanthkumar Kalaiselvan<0xvasanth@gmail.com>"]
 description = "A unified configuration management library for Rust that seamlessly integrates environment variables, config files, and CLI arguments"
 license = "MIT OR Apache-2.0"
-repository = "https://github.com/itsparser/gonfig"
+repository = "https://github.com/0xvasanth/gonfig"
 documentation = "https://docs.rs/gonfig"
-homepage = "https://github.com/itsparser/gonfig"
+homepage = "https://github.com/0xvasanth/gonfig"
 readme = "README.md"
 keywords = ["config", "configuration", "cli", "environment", "settings"]
 categories = ["config", "command-line-interface"]

examples/issue_18_verification.rs

@@ -0,0 +1,233 @@
+use gonfig::{ConfigBuilder, ConfigFormat, Environment, MergeStrategy};
+use serde::{Deserialize, Serialize};
+use std::io::Write;
+use tempfile::NamedTempFile;
+
+/// Multi-level nested configuration structure
+#[derive(Debug, Serialize, Deserialize, PartialEq)]
+struct AppConfig {
+    service: ServiceConfig,
+    http: HttpConfig,
+    database: DatabaseConfig,
+}
+
+#[derive(Debug, Serialize, Deserialize, PartialEq)]
+struct ServiceConfig {
+    name: String,
+    version: String,
+}
+
+#[derive(Debug, Serialize, Deserialize, PartialEq)]
+struct HttpConfig {
+    host: String,
+    port: u16,
+    timeout: u32,
+}
+
+#[derive(Debug, Serialize, Deserialize, PartialEq)]
+struct DatabaseConfig {
+    host: String,
+    port: u16,
+    name: String,
+    pool: PoolConfig,
+}
+
+#[derive(Debug, Serialize, Deserialize, PartialEq)]
+struct PoolConfig {
+    minsize: u32,
+    maxsize: u32,
+}
+
+fn main() -> gonfig::Result<()> {
+    println!("=== Issue #18 Verification: Nested Environment Variable Override ===\n");
+
+    // Create a temporary config file with nested values
+    let mut config_file = NamedTempFile::new().expect("Failed to create temp file");
+    writeln!(
+        config_file,
+        r#"
+service:
+  name: "MyApp"
+  version: "1.0.0"
+
+http:
+  host: "127.0.0.1"
+  port: 3000
+  timeout: 30
+
+database:
+  host: "localhost"
+  port: 5432
+  name: "production_db"
+  pool:
+    minsize: 5
+    maxsize: 20
+"#
+    )
+    .expect("Failed to write config");
+    config_file.flush().expect("Failed to flush");
+
+    println!("1. Loading configuration FROM FILE ONLY:");
+    println!("   Config file path: {:?}", config_file.path());
+
+    let config_file_only: AppConfig = ConfigBuilder::new()
+        .with_merge_strategy(MergeStrategy::Deep)
+        .with_file_format(config_file.path(), ConfigFormat::Yaml)?
+        .build()?;
+
+    println!(
+        "   Service: {} v{}",
+        config_file_only.service.name, config_file_only.service.version
+    );
+    println!(
+        "   HTTP: {}:{} (timeout: {}s)",
+        config_file_only.http.host, config_file_only.http.port, config_file_only.http.timeout
+    );
+    println!(
+        "   Database: {}:{}/{}",
+        config_file_only.database.host,
+        config_file_only.database.port,
+        config_file_only.database.name
+    );
+    println!(
+        "   Pool: min={}, max={}",
+        config_file_only.database.pool.minsize, config_file_only.database.pool.maxsize
+    );
+
+    // Verify file values
+    assert_eq!(
+        config_file_only.http.port, 3000,
+        "File config should have port 3000"
+    );
+    assert_eq!(
+        config_file_only.database.pool.maxsize, 20,
+        "File config should have maxsize 20"
+    );
+
+    println!("\n2. Setting ENVIRONMENT VARIABLES to override nested values:");
+    std::env::set_var("APP_HTTP_PORT", "9000");
+    std::env::set_var("APP_HTTP_TIMEOUT", "60");
+    std::env::set_var("APP_DATABASE_POOL_MAXSIZE", "50");
+    std::env::set_var("APP_DATABASE_NAME", "staging_db");
+
+    println!("   APP_HTTP_PORT=9000");
+    println!("   APP_HTTP_TIMEOUT=60");
+    println!("   APP_DATABASE_POOL_MAXSIZE=50");
+    println!("   APP_DATABASE_NAME=staging_db");
+
+    println!("\n3. Loading configuration WITH NESTED ENV OVERRIDE:");
+    let config_with_env: AppConfig = ConfigBuilder::new()
+        .with_merge_strategy(MergeStrategy::Deep)
+        .with_file_format(config_file.path(), ConfigFormat::Yaml)?
+        .with_env_custom(Environment::new().with_prefix("APP").nested(true))
+        .build()?;
+
+    println!(
+        "   Service: {} v{}",
+        config_with_env.service.name, config_with_env.service.version
+    );
+    println!(
+        "   HTTP: {}:{} (timeout: {}s)",
+        config_with_env.http.host, config_with_env.http.port, config_with_env.http.timeout
+    );
+    println!(
+        "   Database: {}:{}/{}",
+        config_with_env.database.host, config_with_env.database.port, config_with_env.database.name
+    );
+    println!(
+        "   Pool: min={}, max={}",
+        config_with_env.database.pool.minsize, config_with_env.database.pool.maxsize
+    );
+
+    println!("\n4. VERIFICATION RESULTS:");
+
+    // Critical assertion: env vars should override file values
+    if config_with_env.http.port == 9000 {
+        println!("   ✅ PASS: HTTP port overridden by env (9000)");
+    } else {
+        println!(
+            "   ❌ FAIL: HTTP port NOT overridden (expected 9000, got {})",
+            config_with_env.http.port
+        );
+        panic!("Issue #18 NOT FIXED: Environment variable failed to override nested config value");
+    }
+
+    if config_with_env.http.timeout == 60 {
+        println!("   ✅ PASS: HTTP timeout overridden by env (60)");
+    } else {
+        println!(
+            "   ❌ FAIL: HTTP timeout NOT overridden (expected 60, got {})",
+            config_with_env.http.timeout
+        );
+        panic!("Issue #18 NOT FIXED: Environment variable failed to override nested config value");
+    }
+
+    if config_with_env.database.pool.maxsize == 50 {
+        println!("   ✅ PASS: Database pool maxsize overridden by env (50)");
+    } else {
+        println!(
+            "   ❌ FAIL: Database pool maxsize NOT overridden (expected 50, got {})",
+            config_with_env.database.pool.maxsize
+        );
+        panic!("Issue #18 NOT FIXED: Environment variable failed to override deeply nested config value");
+    }
+
+    if config_with_env.database.name == "staging_db" {
+        println!("   ✅ PASS: Database name overridden by env (staging_db)");
+    } else {
+        println!(
+            "   ❌ FAIL: Database name NOT overridden (expected staging_db, got {})",
+            config_with_env.database.name
+        );
+        panic!("Issue #18 NOT FIXED: Environment variable failed to override nested config value");
+    }
+
+    // Verify non-overridden values remain from file
+    assert_eq!(
+        config_with_env.http.host, "127.0.0.1",
+        "Non-overridden values should remain from file"
+    );
+    assert_eq!(
+        config_with_env.service.name, "MyApp",
+        "Non-overridden values should remain from file"
+    );
+    println!("   ✅ PASS: Non-overridden values preserved from config file");
+
+    println!("\n5. Testing WITHOUT nested mode (backward compatibility):");
+    let config_flat: Result<AppConfig, _> = ConfigBuilder::new()
+        .with_merge_strategy(MergeStrategy::Deep)
+        .with_file_format(config_file.path(), ConfigFormat::Yaml)?
+        .with_env_custom(Environment::new().with_prefix("APP").nested(false))
+        .build();
+
+    match config_flat {
+        Ok(cfg) => {
+            println!("   Config loaded with nested=false");
+            println!(
+                "   HTTP port: {} (should be from file: 3000)",
+                cfg.http.port
+            );
+            if cfg.http.port == 3000 {
+                println!(
+                    "   ✅ PASS: Backward compatibility maintained - nested=false keeps flat keys"
+                );
+            }
+        }
+        Err(e) => {
+            println!("   Note: Config might fail without nested mode (expected): {e}");
+        }
+    }
+
+    println!("\n=== CONCLUSION ===");
+    println!("✅ Issue #18 is FIXED!");
+    println!("   Environment variables now properly override nested config file values");
+    println!("   when using .nested(true) with Deep merge strategy.");
+
+    // Clean up
+    std::env::remove_var("APP_HTTP_PORT");
+    std::env::remove_var("APP_HTTP_TIMEOUT");
+    std::env::remove_var("APP_DATABASE_POOL_MAXSIZE");
+    std::env::remove_var("APP_DATABASE_NAME");
+
+    Ok(())
+}