.NET/Java PDF, Tiff, Barcode SDK Library

Migrations are generally preferred over performing SQL statements directly upon the database because they re mostly database independent (Rails outputs the correct SQL for the methods used within migrations for the database engine being used), and the database operations are stored in code rather than being ephemeral operations. Migrations also make it easy to make changes to the database over time. For example, if you wanted to add another column to the entries table, you could simply create a new migration and use add_column and remove_column methods in the up and down methods of that new migration. For example:

winforms pdf 417 reader, winforms qr code reader, winforms upc-a reader, winforms data matrix reader, winforms ean 128 reader, winforms ean 13 reader, c# remove text from pdf, itextsharp replace text in pdf c#, winforms code 39 reader, c# remove text from pdf,

Then you could write 002_add_updated_at_column_to_entries.rb like so:

temporary storage (like the pots and pans of a cooking recipe), but the real power of variables is that you needn t know what values they hold in order to manipulate them.5 For example, you know that x * y evaluates to the product of x and y even though you may have no knowledge of what x and y are. So, you may write programs that use variables in various ways without knowing the values they will eventually hold (or refer to) when the program is run.

2

class AddUpdatedAtColumnToEntries < ActiveRecord::Migration def self.up add_column :entries, :updated_at, :datetime end

Then put that migration into action:

it a presenter simply pours information into the passive minds of the audience. Yet researchers have long known that the mind is not a passive vessel, but rather it is an active participant in the process of learning. It is the minds of your audience that have to create understanding out of the new information they process in working memory. You play an important role in helping your audience create understanding by designing slides in speci c ways that guide the attention of working memory to the most important visual and verbal information, as illustrated in Figure 2-18.

(in /Users/peter/rails/mydiary) == AddUpdatedAtColumnToEntries: migrating ===================================== -- add_column(:entries, :updated_at, :datetime) -> 0.2381s == AddUpdatedAtColumnToEntries: migrated (0.2383s) ============================

You ve seen that you can write programs with variables without knowing their values. Of course, the interpreter must know the values eventually. So how can it be that we don t The interpreter knows only what we tell it, right Not necessarily. You may have written a program, and somebody else may use it. You cannot predict what values they will supply to the program. Let s take a look at the useful function input. (I ll have more to say about functions in a minute.) >>> input("The meaning of life: ") The meaning of life: 42 42 What happens here is that the first line (input(...)) is executed in the interactive interpreter. It prints out the string "The meaning of life: " as a new prompt. I type 42 and press Enter. The resulting value of input is that very number, which is automatically printed out in the last line. Not very useful. But look at the following: >>> x = input("x: ") x: 34 >>> y = input("y: ") y: 42 >>> print x * y 1428 Here, the statements at the Python prompts (>>>) could be part of a finished program, and the values entered (34 and 42) would be supplied by some user. Your program would then print out the value 1428, which is the product of the two. And you didn t have to know these values when you wrote the program, right

The new migration adds an updated_at DATETIME column to the entries table. If you wanted to, you could roll back this migration and go back to the state after the first migration by using this Rake task:

(in /Users/peter/rails/mydiary) == AddUpdatedAtColumnToEntries: reverting ===================================== -- remove_column(:entries, :updated_at) -> 0.0535s == AddUpdatedAtColumnToEntries: reverted (0.0536s) ============================

As described earlier, with the BBP approach, each slide has a headline when you start working in PowerPoint. You then write out what you will say verbally in the off-screen text box in Notes Page view and then add a simple graphic in Normal view to produce a slide, as shown in Figure 2-19.

Note In some circumstances, migrations are created automatically and you simply need to fill them in.

For example, if you use Rails model generator, a migration is automatically created for creating the table associated with the newly generated model. In this chapter, however, we re working in the opposite direction. There s always more than one way to do it!

2

Note This is much more useful when you save your programs in a separate file so other users can execute