Imagine we have a number of related objects (equivalently DB tables), for example:
public class Person {
private String name;
private Date birthday;
private int height;
private Job job;
private House house;
..
}
public class Job {
private String company;
private int salary;
..
}
public class House {
private Address address;
private int age;
private int numRooms;
..
}
public class Address {
private String town;
private String street;
..
}
How to best design a system for easily defining and accessing widely varying subsets of data on these objects/tables? Design patterns, pros and cons, are very welcome. I'm using Java, but this is a more general problem.
For example, I want to easily say:
I'd like some object with (Person.name, Person.height, Job.company, Address.street)
I'd like some object with (Job.company, House.numRooms, Address.town)
Etc.
Other assumptions:
We can assume that we're always getting a known structure of objects on the input, e.g. a Person with its Job, House, and Address.
The resulting object doesn't necessarily need to know the names of the fields it was constructed from, i.e. for subset defined as (Person.name, Person.height, Job.company, Address.street) it can be the array of Objects {"Joe Doe", 180, "ACompany Inc.", "Main Street"}.
The object/table hierarchy is complex, so there are hundreds of data fields.
There may be hundreds of subsets that need to be defined.
A minority of fields to obtain may be computed from actual fields, e.g. I may want to get a person's age, computed as (now().getYear() - Person.birtday.getYear()).
Here are some options I see:
A SQL view for each subset.
Minuses:
They will be almost the same for similar subsets. This is OK just for field names, but not great for the joins part, which could ideally be refactored out to a common place.
Less testable than a solution in code.
Using a DTO assembler, e.g. http://www.genericdtoassembler.org/
This could be used to flatten the complex structure of input objects into a single DTO.
Minuses:
I'm not sure how I'd then proceed to easily define subsets of fields on this DTO. Perhaps if I could somehow set the ones irrelevant to the current subset to null? Not sure how.
Not sure if I can do computed fields easily in this way.
A custom mapper I came up with.
Relevant code:
// The enum has a value for each field in the Person objects hierarchy
// that we may be interested in.
public enum DataField {
PERSON_NAME(new PersonNameExtractor()),
..
PERSON_AGE(new PersonAgeExtractor()),
..
COMPANY(new CompanyExtractor()),
..
}
// This is the container for field-value pairs from a given instance of
// the object hierarchy.
public class Vector {
private Map<DataField, Object> fields;
..
}
// Extractors know how to get the value for a given DataField
// from the object hierarchy. There's one extractor per each field.
public interface Extractor<T> {
public T extract(Person person);
}
public class PersonNameExtractor implements Extractor<String> {
public String extract(Person person) {
return person.getName();
}
}
public class PersonAgeExtractor implements Extractor<Integer> {
public int extract(Person person) {
return now().getYear() - person.getBirthday().getYear();
}
}
public class CompanyExtractor implements Extractor<String> {
public String extract(Person person) {
return person.getJob().getCompany();
}
}
// Building the Vector using all the fields from the DataField enum
// and the extractors.
public class FullVectorBuilder {
public Vector buildVector(Person person) {
Vector vector = new Vector();
for (DataField field : DataField.values()) {
vector.addField(field, field.getExtractor().extract(person));
}
return vector;
}
}
// Definition of a subset of fields on the Vector.
public interface Selector {
public List<DataField> getFields();
}
public class SampleSubsetSelector implements Selector {
private List<DataField> fields = ImmutableList.of(PERSON_NAME, COMPANY);
...
}
// Finally, a builder for the subset Vector, choosing only
// fields pointed to by the selector.
public class SubsetVectorBuilder {
public Vector buildSubsetVector(Vector fullVector, Selector selector) {
Vector subsetVector = new Vector();
for (DataField field : selector.getFields()) {
subsetVector.addField(field, fullVector.getValue(field));
}
return subsetVector;
}
}
Minuses:
Need to create a tiny Extractor class for each of hundreds of data fields.
This is a custom solution that I came up with, seems to work and I like it, but I feel this problem must have been encountered and solved before, likely in a better way.. Has it?
Edit
Each object knows how to turn itself into a Map of fields, keyed on an enum of all fields.
E.g.
public enum DataField {
PERSON_NAME,
..
PERSON_AGE,
..
COMPANY,
..
}
public class Person {
private String name;
private Date birthday;
private int height;
private Job job;
private House house;
..
public Map<DataField, Object> toMap() {
return ImmutableMap
.add(DataField.PERSON_NAME, name)
.add(DataField.BIRTHDAY, birthday)
.add(DataField.HEIGHT, height)
.add(DataField.AGE, now().getYear() - birthday.getYear())
.build();
}
}
Then, I could build a Vector combining all the Maps, and select subsets from it like in 3.
Minuses:
Enum name clashes, e.g. if Job has an Address and House has an Address, then I want to be able to specify a subset taking street name of both. But how do I then define the toMap() method in the Address class?
No obvious place to put code doing computed fields requiring data from more than one object, e.g. physical distance from Address of House to Address of Company.
Many thanks!
Over in-memory object mapping in the application, I would favor database processing of the data for better performance. Views, or more elaborate OLAP/datawarehouse tooling could do the trick. If the calculated fields remain basic, as in "age = now - birth", I see nothing wrong with having that logic in the DB.
On the code side, given the large number of DTOs you have to deal with, you could use classless dynamic (available in some JVM languages) or JSON objects. The idea is that when a data structure changes, you only need to modify the DB and the UI, saving you the cost of changing a whole bunch of classes in between.
I wanted to know if there is a known pattern or convention for the following scenario:
I have two classes: MAT (name:String, address:String) & MATversion(type:String, version:int)
Now I have a DataGrid (DataTable) which will take a generic List of objects for the column mapping and data filling.
The columns should be name, type, version. (Which are distributed in MAT and MATversion)
So I create a class to make this work. This class will merge the needed properties from each class (MAT, MATversion).
-> MAT_MATversion (name:String, type:String, version:int).
Does there exist a naming convention for such an class like MAT_MATversion? Any pattern that mirrors that?
Thanks!
Is there any specific reason why the merged result has to be a unique class?
I'm assuming every MAT object has a single MATversion
you can add a couple of custom properties who will return the type and version of the underlying MATversion object
In C# this would result in something like this
public class MAT{
public String name{ get;set;};
public String adress{ get;set;};
public MATversion myVersion;
public String type {
get{
return myVersion.type;
}
set{
myVersion.type = value;
}
}
public int version {
get{
return myVersion.version;
}
set{
myVersion.version = value;
}
}
}
I'm aware that this doesn't answer the question about design patterns, but I couldn't ask/suggest another approach in a comment since I don't have that right yet.
I have an object in C# that I want to use as a primary key in a database that auto-increments when new objects are added. The object is basically a wrapper of a ulong value that uses some bits of the value for additional hints. I want to store it as a 'pure' ulong value in a database but I would like get an automatic conversion when the value is loaded / unloaded from DB. IE, apply the 'hint' bits to the value based on the table they come from.
I went on a journey of implementing my own IUserType object based on number of examples I found online ( tons of help on this forum ).
I have an ObjectId class that acts is an object ID
class ObjectIdType: IUserType
{
private static readonly NHibernate.SqlTypes.SqlType[] SQL_TYPES = { NHibernateUtil.UInt64.SqlType };
public NHibernate.SqlTypes.SqlType[] SqlTypes
{
get { return SQL_TYPES; }
}
public Type ReturnedType
{
get { return typeof(ObjectId); }
}
...
}
I have a mapping class that looks like this:
public class ObjectTableMap()
{
Id(x => x.Id)
.Column("instance_id")
.CustomType<ObjectIdType>()
.GeneratedBy.Native();
}
At this point I get an exception at config that Id can only be an integer. I guess that makes sense but I was half expecting that having the custom type implemented, the native ulong database type would take over and work.
I've tried to go down the path of creating a custom generator but its still a bit out of my skill level so I am stumbling though it.
My question is, is it possible for me to accomplish what I am trying to do with the mapping?
I think, it is not possible, because your mapping uses the native generator for the Id. This can only be used for integral types (and GUIDs). You can try to use assigned Ids with your custom type, so you are responsible for assigning the values to your Id property.
There is another alternative: Why not set your information bits on class level, instead depending on your table? Your entities represent the tables, so you should have the same information in your entity classes. Example:
class Entity
{
protected virtual ulong InternalId { get; set; } // Mapped as Id
public virtual ulong Id // This property is not mapped
{
get
{
var retVal = InternalId;
// Flip your hint bits here based on class information
return retVal;
}
}
}
You could also turn InternalId into a public property and make the setter protected.
I'm trying to create a dynamic type in .Net.
I want to get a group of Key Value pairs from a DB table and create a new type of object that has a Property-Value relation.
Example:
If I have a row in the table that has a field that says "Licence Plate" and the other field says "STKOVFL". I want to create an object that has a Property called Licence_Plate, and returns the String "STKOVFL".
Is it possible with introspection?
Best Regards!
Here is an very simple example of what your trying to do.
public class ExampleD : DynamicObject
{
public override bool TryGetMember(GetMemberBinder binder, out object result)
{
result = null;
if (binder.Name == "Licence_Plate")
result = "STKOVFL";
return result != null;
}
}
Console.WriteLine(d.License_Plate);
Without using DynaForm and it's kin.
I would like to use a POJO data transfer object, e.g., Person:
public class Person {
private Long id;
private String firstName;
private String lastName;
// ... getters / setters for the fields
}
In the struts live action form we would have:
public class PersonUpdateForm extends SLActionForm {
String organization;
Person[] persons; // all the people will be changed to this organization; they're names and so forth can be updated at the same time (stupid, but a client might desire this)
// getters / setters + index setters / getters for persons
}
What would the corresponding html:text tags look like in the JSP to allow this? If I switch to a List persons field and use a lazy-loading list (in commons-collections) how would that change thinsg?
There seems to be no good way to do this in struts-1.2(.9?)
All help is greatly appreciated!!! If you need more context let me know and I can provide some.
Okay, I believe I've figured it out! The trick is to have your indexed getter create an element each time the getPersons() method is called by the populate method of BeanUtils. The code is completed yet, but I got a positive looking result. It's 3:30 and I've been stuck on this a while. Nobody seemded to know the answer, which makes me want to smack them in the head with a trout. As for my own ignorance ... I only have them to blame!
public List<Person> getPersons() {
persons.add(new Person()); // BeanUtils needs to know the list is large enough
return persons;
}
Add your indexed getters and setters too, of course.
I remember how I actually did this. You must pre-initialize the persons List above to the maximum size you expect to transfer. This is because the List is first converted to an array, the properties then set on each element of the array, and finally the List set back using setPersons(...). Therefore, using a lazy-loading List implementation or similar approach (such as that show above) will NOT work with struts live. Here's what you need to do in more detail:
private List<Person> persons = new ArrayList<Person>(MAX_PEOPLE);
public MyConstructor() { for(int i = 0; i < MAX_PEOPLE; i++) persons.add(new Person()); }
public List<Person> getPeopleSubmitted() {
List<Person> copy = new ArrayList<Person>();
for(Person p : persons) {
if(p.getId() != null) copy.add(p);
// id will be set for the submitted elements;
// the others will have a null id
}
return copy; // only the submitted persons returned - not the blank templates
}
That's basically what you have to do! But the real question is - who's using struts live anymore?!