Tutorial by Examples: co

To remove existing column name from users table, run the command: rails generate migration RemoveNameFromUsers name:string This will generate the following migration: class RemoveNameFromUsers < ActiveRecord::Migration[5.0] def change remove_column :users, :name, :string end end ...

To add a reference to a team to the users table, run this command: $ rails generate migration AddTeamRefToUsers team:references This generates the following migration: class AddTeamRefToUsers < ActiveRecord::Migration[5.0] def change add_reference :users, :team, foreign_key: true ...

To add multiple columns to a table, separate field:type pairs with spaces when using rails generate migration command. The general syntax is: rails generate migration NAME [field[:type][:index] field[:type][:index]] [options] For example, the following will add name, salary and email fields to ...

Say you want to perform in action (in this case, logging "Foo"), while doing something else (logging "Bar"). Normally, if you don't use concurrency, one of these actions is going to be fully executed, and the other run will run only after it's completely finished. But with concur...

// Execute a command a capture standard out. exec.Command creates the command // and then the chained Output method gets standard out. Use CombinedOutput() // if you want both standard out and standerr output out, err := exec.Command("echo", "foo").Output() if err != nil { ...

cmd := exec.Command("sleep", "5") // Does not wait for command to complete before returning err := cmd.Start() if err != nil { log.Fatal(err) } // Wait for cmd to Return err = cmd.Wait() log.Printf("Command finished with error: %v", err)

Generally, the syntax is: SELECT <column names> FROM <table name> WHERE <condition> For example: SELECT FirstName, Age FROM Users WHERE LastName = 'Smith' Conditions can be complex: SELECT FirstName, Age FROM Users WHERE LastName = 'Smith' AND (City = 'New York' OR C...

In a nutshell, conditional pre-processing logic is about making code-logic available or unavailable for compilation using macro definitions. Three prominent use-cases are: different app profiles (e.g. debug, release, testing, optimised) that can be candidates of the same app (e.g. with extra log...

$collection = Mage::getModel('catalog/product')->getCollection(); $collection->addAttributeToFilter('sku', array('like' => 'UX%'));

$collection = Mage::getModel('catalog/product')->getCollection(); // Using operator $collection->addAttributeToFilter('status', array('eq' => 1)); // Without operator (automatically uses 'equal' operator $collection->addAttributeToFilter('status', 1);

When a column name matches a reserved keyword, standard SQL requires that you enclose it in double quotation marks: SELECT "ORDER", ID FROM ORDERS Note that it makes the column name case-sensitive. Some DBMSes have proprietary ways of quoting names. For example, SQL Serve...

Some convenience functions to manipulate data.frames are subset(), transform(), with() and within(). subset The subset() function allows you to subset a data.frame in a more convenient way (subset also works with other classes): subset(mtcars, subset = cyl == 6, select = c("mpg", "...

A popular form of data analysis is split-apply-combine, in which you split your data into groups, apply some sort of processing on each group, and then combine the results. Let's consider a data analysis where we want to obtain the two cars with the best miles per gallon (mpg) for each cylinder cou...

In (standard ANSI/ISO) SQL, the operator for string concatenation is ||. This syntax is supported by all major databases except SQL Server: SELECT 'Hello' || 'World' || '!'; --returns HelloWorld! Many databases support a CONCAT function to join strings: SELECT CONCAT('Hello', 'World'); --retur...

We can provide a consumer that will be called with the multiple relevant values: C++11 template <class F> void foo(int a, int b, F consumer) { consumer(a + b, a - b, a * b, a / b); } // use is simple... ignoring some results is possible as well foo(5, 12, [](int sum, int , int , i...

Run-length encoding captures the lengths of runs of consecutive elements in a vector. Consider an example vector: dat <- c(1, 2, 2, 2, 3, 1, 4, 4, 1, 1) The rle function extracts each run and its length: r <- rle(dat) r # Run Length Encoding # lengths: int [1:6] 1 3 1 1 2 2 # valu...

There are two main ways way to install Ansible on OS X, either using the Homebrew or Pip package manager. If you have homebrew, the latest Ansible can be installed using the following command: brew install ansible To install Ansible 1.9.X branch use following command: brew install homebrew/ver...

type person = {Name: string; Age: int} with // Defines person record member this.print() = printfn "%s, %i" this.Name this.Age let user = {Name = "John Doe"; Age = 27} // creates a new person user.print() // John Doe, 27

The filter or map functions should often be replaced by list comprehensions. Guido Van Rossum describes this well in an open letter in 2005: filter(P, S) is almost always written clearer as [x for x in S if P(x)], and this has the huge advantage that the most common usages involve predicates tha...

The module cmath includes additional functions to use complex numbers. import cmath This module can calculate the phase of a complex number, in radians: z = 2+3j # A complex number cmath.phase(z) # 0.982793723247329 It allows the conversion between the cartesian (rectangular) and polar repr...