Ruby, Rails, fixtures and fails

After a long time working on top of Java and its beautiful JVM and robust ecosystem my professional path lead me to another powerful web development system – Ruby and Rails. So how did I felt during the ‘transformation’ or did I metamorphose into a vermin ?

From programmsers point of view, very comfortable. Ruby was/is too easy to learn and grasp especially if you have worked with Groovy in the last couple of years. On the other hand Rails paradigms are very close and similar to that of Grails (it’s MVC and it was inspired by Rails so) and most of my web development is based on it. However changing the language and the framework happened to be more challenging from another aspect. I’m talking about the tools, helpers, libraries/gems and what the community have built until now in the field of helping and improving the development processes. Both Groovy & Grails and Ruby & Rails are open source, so you know what I mean.

As I dived more deeply into the presented problems I found about a feature that makes Rails great tool for TDD (Test Driven Development) – fixtures. Basically, a set of YAML files that contain mock data that can be loaded during the testing process. That means tables, rows, associations presented in a YAML format. Pretty cool.

But the transformation from YAML data into RDS principles for storing data isn’t so simple. One of it’s issues is the foreign key references. The problem that occurred for me with fixtures is the loading order of the files and their mapping into a database. Let’s assume that we have three tables: items, users, and user_items and the latter contains references to both user and item table. With fixtures we will have three different files: users.yml, items.yml, and user_items.yml that contain the mock data. With the start of the test process the Active Record fixture module starts loading this files into memory and executes the queries responsible for inserting data into the database. And everything is cool, no problem, but happens if we first have the user_items.yml loaded into the DB? Well it will fail. Why? Because of the database referential constraint system we will face the problem of non existing foreign key values for the user and item references.

Well Rails developers weren’t so stupid and they thought of this. If you dive into the fixtures implementation it will be revealed that Rails is invoking the method disable_referential_integrity. That means that Rails will try to remove the constraints for the test database and just insert the data. But on most RDS system the database users need to have super user privileges to execute those commands.

Since I stumbled upon on this problem and it reflected both locally and on the CI system, I needed to find a ‘workaround’ solution. So I started thinking that if we control the order of loading of the YAML files then we can control the inserting flow and like a consequence bypass the logical problem of referential integrity (the default loading is randomly alphabetical, but I’m not sure). Then started googling, reading blogs, scanning through stackoverflow and you know all of that monkey attempts to solve your problem. And luckily then – eureka! I found the solution: override fixtures method for loading yaml files and control the order of deleting the data (since that is influenced by the referential constraint too).

The solution is simple and can be found in this gist. Extending the ActiveSupport:FixtureSet class with the purpose to override the method create_fixtures (but not totally reimplement it thanks to the help of ruby aliases) that is responsible for the obvious, creating the fixtures, nails it. We implement this code sample in a file that is required by the tests. With the UserItem.delete_all we care that all user items are first deleted before the User and Item tables are dropped. The variable fs_names holds the names of the items/files that will be loaded and gives priority to users and items before any other. That means that they will be processed and loaded before the user_items yaml file and there won’t be any referential integrity issue.

Till I came to this which is just modified version of a proposed solution in the stackoverflow network I read and peeked into a lot of online resources. With this in mind I hope someone with similar problem will face this blog post first and save his/her time for something more productive that scraping half of the internet for a solution.

Cheers, I.


Used resources:

Deciphering an API documentation

JavaScript on the Web has a lot of APIs to work with. Some of them are fully supported some are still in draft. One of the things I worked with lately is the FileSystemDirectoryReader Interface of the File and Directory Entries API. It defines only one method called readEntries that returns an array containing some number of the directory’s entries. This is draft proposed API and is not supposed to be fully browser compatible. However I’ve tested it almost on all the latest versions of browsers and it works fine on each one except for the Safari browser (I think ;)).

This post will focus on one example that shows that sometimes reading an API documentation can be a little tricky. So the example in the documentation shows a common use of this API where the source of the FileSystemEntry items that we read from are passed with a (drag and) drop event in the browser. The FileSystemEntry can be either a file or a directory. What we want to do is build a file system tree of the dropped item . If the dropped item is a directory then the item is actually FileSystemDirectoryEntry object than defines the createRender method that creates the FileSystemDirectoryReader object on which we will call the readEntries method.

The demo example can be tested in this fiddle. What I want you to do is to drop a directory that contains more than 100 files. If you do that you can notice that the readEntries method returns only the first 100 queued files. That is the main reason for writing this post. The description on the successCallback argument of the readEntries method is a little bit confusing, it says: “A function which is called when the directory’s contents have been retrieved. The function receives a single input parameter: an array of file system entry objects, each based on FileSystemEntry. Generally, they are either FileSystemFileEntry objects, which represent standard files, or FileSystemDirectoryEntry objects, which represent directories. If there are no files left, or you’ve already called readEntries() on this FileSystemDirectoryReader, the array is empty.”

In their example we can see the scanFiles method that reads the items and creates html elements:

function scanFiles(item, container) {
        var elem = document.createElement("li");
        elem.innerHTML = item.name;
        container.appendChild(elem);

        if (item.isDirectory) {
            var directoryReader = item.createReader();
            var directoryContainer = document.createElement("ol");
            container.appendChild(directoryContainer);

            directoryReader.readEntries(function (entries) {
                entries.forEach(function (entry) {
                    scanFiles(entry, directoryContainer);
                });
            });
        }
    }

It seems that the successCallback functions returns the entries partially in a packages of 0 to max 100 items.  If we use this function we will never iterate more that 100 items in the given directory. What we need to do is to decipher this part: “If there are no files left, or you’ve already called readEntries() on this FileSystemDirectoryReader, the array is empty.”.  Translated this into JavaScript code is:

function scanFiles(item, container) {
        var elem = document.createElement("li");
        elem.innerHTML = item.name;
        container.appendChild(elem);

        if (item.isDirectory) {
            var directoryReader = item.createReader();
            var directoryContainer = document.createElement("ol");
            container.appendChild(directoryContainer);

            var fnReadEntries = (function () {
                return function (entries) {
                    entries.forEach(function (entry) {
                        scanFiles(entry, directoryContainer);
                    });
                    if (entries.length > 0) {
                        directoryReader.readEntries(fnReadEntries);
                    }
                };
            })();

            directoryReader.readEntries(fnReadEntries);
        }
    }

The change that we need to apply is to check after the iteration of the entries if the length of the entries array is bigger than 0.  Case that’s the truth then we should call the readEntries method again. If the entries size is zero, then the iteration is finished – all the file system items are iterated.

This fiddle has the improved version of the scan file method that will list all the files in the directory (more that 100), and won’t trick you.

Now finally we can conclude what the part “returns an array containing some number of the directory’s entries” meant. 🙂

Cheers

A Groovy parser for CSV files

Parsing CSV these days is pretty straight-forward and not a big deal especially when we have the handy libraries from Apache Commons (I’m talking bout Java world). In this post I will give you an example how to use the Apache Commons CSV with the magic of Groovy and its closures so it can look and feel a little more fun because parsing in general is job for sad people (not kidding).

We’ll make ourself a simple Groovy class that will hold a reference to the CVSParser file, and a reference to the headers and the current record/line of the file that we will iterate with the closure delegate set to the instance of this CSVParserUtils class.

Something like this:

class CSVParseUtils {

    CSVParser csvFile
    def record
    def headers

    CSVParseUtils(String fileLocation) {
        def reader = Paths.get(fileLocation).newReader()
        CSVFormat format = CSVFormat.DEFAULT.withHeader().withDelimiter(delimiter)
        csvFile = new CSVParser(reader, format)
        def header = csvFile.headerMap.keySet().first()
        headers = header.split(delimiter as String)
    }

As we can see it’s a constructor that takes the location to the CSV file that we want to parse, creates some default parsing format and generates new CSVFile that holds the CSV data.

As we see parsing is easy, but it’s better when we can transform the data on the run as we loop it. For that reason we will define a method called eachLine that will take a params Map and a Closure that will have access to the record/line instance and will do something with it.

/**
 * List each line of the csv and execute closure
 * @param params
 * @param closure
 */
def eachLine(Map params = [:], Closure closure) {
    def max = params.max ?: maxLines
    int linesRead = 0
    def rowIterator = csvFile.iterator()
    closure.setDelegate(this)

    while (rowIterator.hasNext() && linesRead++ < max) {
        record = rowIterator.next()
        closure.call(record)
    }
}

It’s nothing special only a simple loop that iterates through the iterator and calls the closure with the given record for that line as a closure argument.

How to use it?

def parser = new CSVParseUtils(fileLocation)
def result = [:]
// first 2 lines without header
parser.eachLine([max: 2]) { CSVRecord record ->
    result.put(record.recordNumber, record.values.size() > 4 ? 
             record.values[0..4] : record.values[0..record.values.size()])
}

We imagine that we need only the first 2 lines and the first 5 columns or something like that.

As you can see this closure loop is not specially connected with CSV, it’s just a clean way to iterate through any textual file line by line and do something with it. As a matter of fact you can use the BufferedReader which has method eachLine too.

The source code for this whole example can be found on github.

Thanks for reading.

Grails and SAPUI5 are friends

Hello reader,

instead of the planned walk through the city park and drinking some beer(s) mother nature swinging moods changed my plans and in place of the shiny sun gave me hard rain and sour mouth. In a situation like that, alone and bored I decided to bore you too and share this short text about two good friends called Grails and SAPUI5 (respect to the OpenUI5 project too). 🙂

I’ve been working hard with the Grails framework this couple of years and different situations led me to different scenarios. Lately I found myself in situation that asked bringing closer the powerful SAP services to the web/mobile clients. And what is better that using the outsourced JavaScript MVC framework made by SAP called SAPUI5 or if you prefer the open source project name OpenUI5 in conjunction with the versatile Grails Framework.

If you’re familiar with Grails then you certainly know that with the latest 3++ versions of Grails there is great support for already established and pretty much famous frameworks/libraries AngularJS and ReactJS in forms of Grails app profiles and plugins. But there is no “official” support for interbridging SAPUI5 and Grails and that is the main motive for writing this blog post and sharing it with you.

SAPUI5 is a single paged application where all the magic is done with JS so what we need is a single html file or in this case a single gsp file. We use that file to define the paths to the SAPUI5 runtime (or sdk) resources and to init the main SAPUI5 application via short JavaScript code. SAPUI5 is best when used with the OData services and that its where this software shines, however it has also great support when working with JSON and provides us with swift JSONModel that we can use to fill up the application data. And because we have JSON then we must have the Restful Grails controllers that will provide us with well defined JSON.

So the situation is pretty simple: Grails connects us with the backend via web services (or else?) or it provides us the data on its own via GORM or something else. Then Grails transforms the data into a JSON format that is a sweet cake for the SAPUI5 to consume and make it look great both on web browsers and on any mobile clients (Smart phones, tablets etc.).

Well this won’t be worth a penny without a working example, right? Because that’s the cause I’ve published a little demo of Grails and SAPUI5 playing together that you can check it on github. In short words we have a Spring Security Core plugin for the authentication and authorization, the JSON Views plugin for making the JSON even easier and also an example how to make it work via rest based http calls if your clients is native app . And of course the SAPUI5 application itself.

Here’s the link to the repo.

Thanks for reading,

cheers.

 

 

Passing by ‘reference’ in Java

One of the first things that every Java programmer learns (or should learn) is that method parameters in Java are passed-by-value. That is the only truth and there is no so called ‘reference’ passing in Java. Every method call with parameters means that their value is copied in some memory chunk and then they are passed (the copied memory) to the local function to be used.

What is more important though is of what type is the parameter that is passed. Generally there are two different data types: primitive (int, char, double etc…) and complex aka objects (Object, array etc…). The thing that matters is what is the ‘value’ of them when they are used in parameter passing.

When we are passing parameters of primitive type we are passing the actual value of it. So if we pass an integer with value of 4, then the function will receive an integer with value of 4 as parameter value. However, if we pass parameter of complex type let’s say some object of class Company then the function will actually receive the pointer to real object location in memory. Or in Java terms, it will receive the copied value of the reference (address) to the Java object that we want to pass and use.

If in C++ we have: Company *c; to get the pointer , then in Java we have Company c; . It’s pretty much the same, the difference is how things are designed and implemented under the cover.

If we understand this, then we realize that even if there are no out parameters in Java when defining and implementing methods, we can still use the reference advantage to program that thing by our self.

To get things clear and imagine the picture we should actually see the picture, I mean the code.

For example we can use an array of size one to be our data holder. Passing and mutating data this way will change the real value that we want to be changed. An example code, try it:


package com.groggystuff;

/**
*
* @author Igor
*/
public class JavaArguments {

/**
*
* @param argument
*/
public static void mutate(int argument){
argument++;
}

/**
*
* @param argument
*/
public static void mutate(int[] argument){
argument[0]++;
}

/**
* @param args the command line arguments
*/
public static void main(String[] args) {
// example pass by value
int i = 4;
System.out.println("Value of 'i' before mutation: "+i); //prints 4
mutate(i);
System.out.println("Value of 'i' after mutation: "+i); // prints 4

// example pass by value too
int[] j = new int[1];
j[0] = 4;
System.out.println("Value of 'j' before mutation: "+j[0]); // prints 4
mutate(j);
System.out.println("Value of 'j' after mutation: "+j[0]); // prints 5

}

}

Thanks for reading,

I hope this post will be helpful to you in solving the coding mysteries in life, or something similar.

Please comment if you feel that  your comment is needed. Or comment at will, just to say hi for example.