When we designing data models and their corresponding tables appears sometimes Boolean as datatype. In general those flags are not really problematic. But maybe there could be a better solution for the data design. Let me give you a short example about my intention.
Assume we have to design a simple domain to store articles. Like a Blog System or any other Content Management. Beside the content of the article and the name of the author could we need a flag which tells the system if the article is visible for the public. Something like published as a Boolean. But there is also an requirement of when the article is scheduled a date for publishing. In the most database designs I observed for those circumstances a Boolean: published and a Date: publishingDate. In my opinion this design is a bit redundant and also error prone. As a fast conclusion I would like to advice you to use from the beginning just Date instead of Boolean. The scenario I described above can also transformed to many other domain solutions.
For now, after we got an idea why we should replace Boolean for Date datatype we will focus about the details how we could reach this goal.
Dealing with standard SQL suggest that replacing a Database Management System (DBMS) for another one should not be a big issue. The reality is unfortunately a bit different. Not all available data types for date like Timestamp are really recommendable to use. By experience I prefer to use the simple java.util.Date to avoid future problems and other surprises. The stored format in the database table looks like: ‘YYYY-MM-dd HH:mm:ss.0’. Between the Date and Time is a single space and .0 indicates an offset. This offset describes the time zone. The Standard Central European Timezone CET has an offset of one hour. That means UTC+01:00 as international format. To define the offset separately I got good results by using java.util.TimeZone, which works perfectly together with Date.
Before we continue I will show you a little code snippet in Java for the OR Manager Hibernate and how you could create those table columns.
@Table(name = "ARTICLE")
public class ArticleDO {
@CreationTimestamp
@Column(name = "CREATED")
@Temporal(TemporalType.DATE)
private Date created;
@Column(name = "PUBLISHED")
@Temporal(TemporalType.DATE)
private Date published;
@Column(name = "DEFAULT_TIMEZONE")
private String defaultTimezone;
//Constructor
public ArticleDO() {
TimeZone.setDefault(Constraints.SYSTEM_DEFAULT_TIMEZONE);
this.defaultTimezone = "UTC+00:00";
this.published = new Date('0000-00-00 00:00:00.0');
}
public Date isPublished() {
return published;
}
public void setPublished(Date publicationDate) {
if(publicationDate != null) {
this.published = publicationDate;
} else {
this.published = new Date(System.currentTimeMillis());
}
}
} -- SQL
INSERT INTO ARTICLE (CREATED, PUBLISHED, DEFAULT_TIMEZONE)
VALUES ('1984-04-01 12:00:01.0', '0000-00-00 00:00:00,0', 'UTC+00:00);Let get a bit closer about the listing above. As first we see the @CreationTimestamp Annotation. That means when the ArticleDO Object got created the variable created will initialized by the current time. This value never should changed, because an article can just once created but several times changed. The Timezone is stored in a String. In the Constructor you can see how the system Timezone could grabbed – but be careful this value should not trusted to much. If you have a user like me traveling a lot you will see in all the places I stay the same system time, because usually I never change that. As default Timezone I define the correct String for UTC-0. The same I do for the variable published. Date can also created by a String what we use to set our default zero value. The Setter for published has the option to define an future date or use the current time in the case the article will published immediately. At the end of the listing I demonstrate a simple SQL import for a single record.
But do not rush to fast. We also need to pay a bit attention how to deal with the UTC offset. Because I observed in huge systems several times problems which occurred because developer was used only default values.
The timezone in general is part of the internationalization concept. For managing the offset adjustments correctly we can decide between different strategies. Like in so many other cases there no clear right or wrong. Everything depends on the circumstances and necessities of your application. If a website is just national wide like for a small business and no time critical events are involved everything become very easy. In this case it will be unproblematic to manage the timezone settings automatically by the DBMS. But keep in mind in the world exist countries like Mexico with more than just one timezone. An international system where clients spread around the globe it could be useful to setup each single DBMS in the cluster to UTC-0 and manage the offset by the application and the connected clients.
Another issue we need to come over is the question how should initialize the date value of a single record by default? Because null values should avoided. A full explanation why returning null is not a good programming style is given by books like ‘Effective Java’ and ‘Clean Code’. Dealing with Null Pointer Exceptions is something I don’t really need. An best practice which well works for me is an default date – time value by ‘0000-00-00 00:00:00.0’. Like this I’m avoiding unwanted publishing’s and the meaning is very clear – for everybody.
As you can see there are good reasons why Boolean data types should replaced by Date. In this little article I demonstrated how easy you can deal with Date and timezone in Java and Hibernate. It should also not be a big thing to convert this example to other programming languages and Frameworks. If you have an own solution feel free to leave a comment and share this article with your colleagues and friends.
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