1_houserest.cls

@RestResource(urlMapping='/WarehouseREST/*')
global with sharing class WarehouseREST {

	global WarehouseREST() {}
}

2_houserest.cls

	@HttpPost 
	global static String postMerchandiseToInvoice(Id invoiceID, List<Id> merchIDs) {
		List<Line_Item__c> line_items = new List<Line_Item__c>();
		
		for(Id m : merchids) {
			Line_Item__c li = new Line_Item__c(Invoice__c = invoiceID, Merchandise__c=m, Quantity__c=1);
			line_items.add(li);
		}
		try {
			insert line_items;
			return line_items.size() +' item(s) added';
		} catch (DMLException e) {
			return e.getMessage();
		}		
	}

3_houserest.cls

	@HttpGet
	global static Map<String,Integer> getMerchandiseForInvoice() {
		String id = RestContext.request.requestURI.substring(RestContext.request.requestURI.lastIndexOf('/')+1);
		List<AggregateResult> lis = [SELECT SUM(Line_Item_Total__c) total, Merchandise__r.Name name from Line_Item__c where Invoice__c = :id Group By Merchandise__r.Name];
		
		Map<String,Integer> results = new Map<String,Integer>();
		for(AggregateResult a : lis) {
			results.put(String.valueOf(a.get('name')),Integer.valueOf(a.get('total')));
		}
		
		return results;
	}

4_houserest.cls

	@HttpDelete
	global static String deleteMerchandiseFromInvoice() {
		Id merchId = RestContext.request.params.get('mid');
		Id invoiceId = RestContext.request.params.get('iid');
		List<Line_Item__c> line_items = [SELECT ID from Line_Item__c WHERE Merchandise__c = :merchID AND Invoice__c = :invoiceId];
		
		try {
			delete line_items;
			return 'Done';
		} catch (DMLException e) {
			return 'Error';
		}
	}

5_houserest.cls

	global class RESTMessage {
		public String message {get; set;}
		public String status {get; set;}
		public String info {get; set;}
		public RESTMessage(String m, String s, String i) {
			this.message = m;
			this.status = s;
			this.info = i;
		}
	}

6_houserest.cls

		try {
			insert line_items;
			return new RESTMEssage(line_items.size() +' item(s) added','Completed', invoiceId);
		} catch (DMLException e) {
			return new RESTMEssage(e.getMessage(),'Error', invoiceId);
		}		

7_houserest.cls

@RestResource(urlMapping='/WarehouseREST/*')
global with sharing class WarehouseREST {

	global WarehouseREST() {}
	
	@HttpGet
	global static List<AggregateResult> getMerchandiseForInvoice() {
		String id = RestContext.request.requestURI.substring(RestContext.request.requestURI.lastIndexOf('/')+1);
		List<AggregateResult> lis = [SELECT SUM(Line_Item_Total__c), Merchandise__r.Name from Line_Item__c where Invoice__c = :id Group By Merchandise__r.Name];
		return lis;
	}
	
	@HttpPost 
	global static RESTMessage postMerchandiseToInvoice(Id invoiceID, List<Id> merchids) {
		List<Line_Item__c> line_items = new List<Line_Item__c>();
		
		for(Id m : merchids) {
			Line_Item__c li = new Line_Item__c(Invoice__c = invoiceID, Merchandise__c=m,Quantity__c=1);
			line_items.add(li);
		}
		try {
			insert line_items;
			return new RESTMEssage(line_items.size() +' item(s) added','Completed', invoiceId);
		} catch (DMLException e) {
			return new RESTMEssage(e.getMessage(),'Error', invoiceId);
		}		
	}

	
	@HttpDelete
	global static String deleteMerchandiseFromInvoice() {
		Id merchId = RestContext.request.params.get('mid');
		Id invoiceId = RestContext.request.params.get('iid');
		List<Line_Item__c> line_items = [SELECT ID from Line_Item__c WHERE Merchandise__c = :merchID AND Invoice__c = :invoiceId];
		
		try {
			delete line_items;
			return 'Done';
		} catch (DMLException e) {
			return 'Error';
		}
	}
	
	global class RESTMessage {
		public String message {get; set;}
		public String status {get; set;}
		public String info {get; set;}
		public RESTMessage(String m, String s, String i) {
			this.message = m;
			this.status = s;
			this.info = i;
		}
	}


}

8_houserest.cls

 @isTest
global class WarehouseRESTTest {
    @IsTest
    global static void testWarehouseREST() {    
        Warehouse__c w = new Warehouse__c(Name='Nowheresville'); 
        insert w;
        Merchandise__c m = new Merchandise__c(Name='Rack Server',Price__c=1245.99,Quantity__c=500,
                                                Warehouse__c = w.Id);
        insert m;
        
        Invoice__c i = new Invoice__c();
        insert i;
        
        
        RestRequest req = new RestRequest(); 
        RestResponse res = new RestResponse();
        
        WarehouseREST.RESTMessage r = WarehouseREST.postMerchandiseToInvoice(i.id,new Id[]{m.id});
        System.assertEquals(r.status,'Completed');
        
        // pass the req and resp objects to the method       
        req.requestURI = 'https://www.salesforce.com/services/apexrest/WarehouseREST/'+i.Id;  
        req.httpMethod = 'GET';
        RestContext.request = req;
        RestContext.response = res;
        
        Map<String,Integer> get_results = WarehouseREST.getMerchandiseForInvoice();
        System.assertEquals(get_results.size(),1);
        
        // pass the req and resp objects to the method       
        req.requestURI = 'https://www.salesforce.com/services/apexrest/WarehouseREST/?iid='+i.Id+'&'+m.id;  
        req.httpMethod = 'DELETE';
        RestContext.request = req;
        RestContext.response = res;
        
            String response = WarehouseREST.deleteMerchandiseFromInvoice();
            System.assertEquals('Done',response);
        
        
    }
}