ActiveRecord: List columns in table from console

Question

I know that you can ask ActiveRecord to list tables in console using:

ActiveRecord::Base.connection.tables

Is there a command that would list the columns in a given table?

Solution 1

This will list the column_names from a table

Model.column_names
e.g. User.column_names

Solution 2

This gets the columns, not just the column names and uses ActiveRecord::Base::Connection, so no models are necessary. Handy for quickly outputting the structure of a db.

ActiveRecord::Base.connection.tables.each do |table_name|
  puts table_name
  ActiveRecord::Base.connection.columns(table_name).each do |c| 
    puts "- #{c.name}: #{c.type} #{c.limit}"
  end
end

Sample output: http://screencast.com/t/EsNlvJEqM

Solution 3

Using rails three you can just type the model name:

> User
gives:
User(id: integer, name: string, email: string, etc...)

In rails four, you need to establish a connection first:

irb(main):001:0> User
=> User (call 'User.connection' to establish a connection)
irb(main):002:0> User.connection; nil #call nil to stop repl spitting out the connection object (long)
=> nil
irb(main):003:0> User
User(id: integer, name: string, email: string, etc...)

Solution 4

If you are comfortable with SQL commands, you can enter your app's folder and run rails db, which is a brief form of rails dbconsole. It will enter the shell of your database, whether it is sqlite or mysql.

Then, you can query the table columns using sql command like:

pragma table_info(your_table);

Solution 5

complementing this useful information, for example using rails console o rails dbconsole:

Student is my Model, using rails console:

$ rails console
> Student.column_names
 => ["id", "name", "surname", "created_at", "updated_at"] 

> Student
 => Student(id: integer, name: string, surname: string, created_at: datetime, updated_at: datetime)

Other option using SQLite through Rails:

$ rails dbconsole

sqlite> .help

sqlite> .table
ar_internal_metadata  relatives             schools             
relationships         schema_migrations     students 

sqlite> .schema students
CREATE TABLE "students" ("id" integer PRIMARY KEY AUTOINCREMENT NOT NULL, "name" varchar, "surname" varchar, "created_at" datetime NOT NULL, "updated_at" datetime NOT NULL);

Finally for more information.

sqlite> .help

Hope this helps!

View more solutions

Comments

[owl]

OP just wants the column names.

[Yule]

Perhaps. But not necessarily. It is an alternative way of getting them with extra info that is sometimes helpful when listing columns from console

[Andrew]

This is also a useful method to know, IMO. @Yule - does this query the schema/migrations code etc. or does it query the DB? The reason I ask is I was experiencing a mismatch between my schema and what actually was in the DB (one migration glitched), so specifically I needed to be sure I was seeing what was actually in the table.

[srt32]

You could also run something like Model.columns to get more info about the columns including database config data.

[sayap]

In rails 3.2, doing it this way somehow doesn't set the primary attribute correctly (all columns have primary=nil). It is set correctly with the Model.columns method suggested by srt32.

[Yule]

@Andrew it queries the DB (hence the need to establish a connection in rails 4)

[nibbex]

Great! Using Model.columns provides all the information for a table through ActiveRecord. Crucially for me it was the only and easiest way to gain confidence in what my primary key really was at the database level.

[AJFaraday]

You could always use Model.primary_key, which gives you the name of the primary key according to rails. (This will be 'id' unless it's declared in the model as something else).

[valk]

For mySQL use describe your_table;, not perfect but works

[oklas]

You can use online database console (gem activeadmin-sqlpage) as describerd in this answer if you use active admin.

[Pak]

Does not exist in rails 5+

[baash05]

This is the right answer. There is no requirement to have a model. Not every table has a model. "has_many_and_belongs_to"

[Richard_G]

To provide a list sorted by table name, use: ActiveRecord::Base.connection.tables.sort.each

[A moskal escaping from Russia]

The answer doesn't state that, and the question is tagged Rails 3 @Pak...

[Pak]

@SebastianPalma I know, but as this is a well-referenced question such comments are imo helpful for people looking for answers, fast. The tag was probably chosen during the initial write-up but the question is quite generic.