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Figure 9-9. Getting total amount paid by employee This method is not without its problems, however. The primary issue is that it is quite tedious to add new payslips to the document, because they would need to be inserted into the existing employee record structure. It can also lead to an increase in conflict errors. If someone tries to modify an employee s details, for example, while another user is inserting a payslip into the record, they will get a 409 Conflict error. An alternative method of storing related data is to store it in a separate document; in that case, each employee would have one employee document and zero or more payslip documents. To make it easy to distinguish one from the other, you might create a type field in each document to indicate whether it is an employee document or a payslip document. The payslip document would also need to store a reference to the employee to which it belongs. Using this method, the employee document would look like the following: { type: "employee", employee_no: 1, name: "David Harrington",

When you locate an item you were searching for, simply tap it. The store moves you to the album page that lists that track. If more than one exact match exists, it displays a grouped list of songs and albums showing the matched items.

var book = new Object();

salary: 31000, department: "IT" } It would then have three separate payslip documents: { type: "payslip", employee_no: 1, date: "20090825", amount: 2100 } { type: "payslip", employee_no: 1, date: "20090725", amount: 2100 } { type: "payslip", employee_no: 1, date: "20090625", amount: 2100 } Adding new payslips is far easier with this method, because you simply create a new document. It also doesn t suffer from concurrency issues, because the employee and payslip data are stored independent of one another so one user can change the employee record at the same time as another user changes a payslip record. Getting the total amount paid for each employee is simple, as shown in Listing 9-11. Listing 9-11. Getting the Total Amount Paid by Employee map: function(doc) { if(doc.type == "payslip") emit(doc.employee_no, doc.amount); } reduce: function(keys, values, rereduce) { return sum(values); } The obvious downside to this method is that you would need to use a separate view to get back the employee s information, such as their name. As a result, a separate HTTP request is required, adding load to the server. Fortunately, you can use view collation to bring back documents of different types alongside each other. So, you could bring back the employee record and then all of their associated payslip documents after it. To do this, you would use what is known as a complex key, using the employee_no field and an identifier to say which type of document should appear first. Listing 9-12 shows the code.

In the iTunes Wi-Fi Music Store, tap the name of any song to listen to a preview. You ll see the track number or album image next to the name flip around to show the playback control, as shown in Figure 7-6.

As you ll discover in section 3.1.4, the new operator is useful when you want to create custom objects using a function as a constructor. In the previous statement, the new operator is syntactic sugar, because an object is usually created with an object literal, like so:

Listing 9-12. Using View Collation to Join Documents map: function(doc) { if(doc.type == "employee") emit([doc.employee_no, 0], doc); else if(doc.type == "payslip") emit([doc.employee_no,1], doc); } This will first and foremost return the document containing the employee s information, followed by any payslip documents associated with that employee. You can see this in Figure 9-10.

Figure 7-6. Each song listing allows you to play a 30-second preview and lists the purchase price for the track.

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Figure 9-10. Using view collation to join documents Which method is best really depends on the data you are working with. If your related data has many fields and requires constant additions and updates, you are probably better off storing this data in separate documents.