mysqldump multiple/selected tables
This post explains how to dump selected tables only.
The actual syntax being:
mysqldump [ -u ] DATABASE_NAME [TABLES_LIST] > FILE_NAME
So, to dump the entire database use,
mysqldump -u USER_NAME DATABASE_NAME > FILE_NAME
To dump only selected tables use,
mysqldump -u USER_NAME DATABASE_NAME TABLE_1 TABLE_3 TABLE_4 TABLE_9 > FILE_NAME
pidgin yahoo login solution
Recently there has been a pidgin-yahoo connect/login issue.
Had to connect thru yahoo-web-messenger for two days until one of my friend figured out, probably from another web source.
The fix is :
All you need to do is add a line at the end of /etc/hosts file.
The line is :
66.163.181.170 scs.msg.yahoo.com
vim /etc/hosts
66.163.181.170 scs.msg.yahoo.com
Mysql gem installation fix in rails
Today when i was trying to install mysql gem on the rails 2.2.2 + ruby 1.8.6 + gem 1.2.0 on FEDORA, i had the following error:
———————————————————————————————–
Building native extensions. This could take a while…
ERROR: Error installing mysql:
ERROR: Failed to build gem native extension.
/usr/bin/ruby extconf.rb install mysql — —with-mysql-config=/usr/local/mysql/bin/mysql_config
checking for mysql_query() in -lmysqlclient… no
checking for main() in -lm… yes
checking for mysql_query() in -lmysqlclient… no
checking for main() in -lz… yes
checking for mysql_query() in -lmysqlclient… no
checking for main() in -lsocket… no
checking for mysql_query() in -lmysqlclient… no
checking for main() in -lnsl… yes
checking for mysql_query() in -lmysqlclient… no
*** extconf.rb failed ***
Could not create Makefile due to some reason, probably lack of
necessary libraries and/or headers. Check the mkmf.log file for more
details. You may need configuration options.
Provided configuration options:
–with-opt-dir
–without-opt-dir
–with-opt-include
–without-opt-include=${opt-dir}/include
–with-opt-lib
–without-opt-lib=${opt-dir}/lib
–with-make-prog
–without-make-prog
–srcdir=.
–curdir
–ruby=/usr/bin/ruby
–with-mysql-config
–without-mysql-config
–with-mysql-dir
–without-mysql-dir
–with-mysql-include
–without-mysql-include=${mysql-dir}/include
–with-mysql-lib
–without-mysql-lib=${mysql-dir}/lib
–with-mysqlclientlib
–without-mysqlclientlib
–with-mlib
–without-mlib
–with-mysqlclientlib
–without-mysqlclientlib
–with-zlib
–without-zlib
–with-mysqlclientlib
–without-mysqlclientlib
–with-socketlib
–without-socketlib
–with-mysqlclientlib
–without-mysqlclientlib
–with-nsllib
–without-nsllib
–with-mysqlclientlib
–without-mysqlclientlib
Gem files will remain installed in /usr/lib/ruby/gems/1.8/gems/mysql-2.7 for inspection.
Results logged to /usr/lib/ruby/gems/1.8/gems/mysql-2.7/gem_make.out
ERROR: could not find gem — locally or in a repository
ERROR: could not find gem —with-mysql-config=/usr/local/mysql/bin/mysql_config locally or in a repository
————————————————————————————————-
The fix for it is:
gem install mysql -- --with-mysql-config=/usr/bin/mysql_config
Tats it. Its working fine for me.
Hope it works 4 oder folks too.
If not, it means that a library called mysql-devel is missing.
So, install it by “sudo yum install mysql-devel” or “sudo apt-get install libmysql++-dev”
And then, install the mysql-gem. This should for sure fix it.
For further info, checkout: Rails Mysql wiki
Undefining class at runtime
This post explains undefining the class at runtime.
Im not gonna explain the live-cases where this might be used. Ask Uncle Google for that.
Lets jump in at d code directly.
When u create a new class, say “Person”, two things happen:
- A new object of type(class) Class is instantiated.
- A new constant called Person will be instantiated and will hold the value of the class-object created above.
Proof: Class Person;end;
Try, Person.class #=> Class. This means that the Person holds a value which is of class Class. (1 is proved).
Try, Person = 123 #=> Curses saying, Person is already initialized, which means Person is a constant. (2 is proved).
Now, All the constants(Person) and other are silently ducked into the class Object.
Proof: Object.class_eval{Person} #=> Person Class
So, we should understand by now that the newly created class Person resides as a constant of the class Object.
So if we could somehow remove that constant, then the class Person will also be removed.
There is a method called remove_const for the class Object.
So, try Object.remove_const : Person #=> Curses, NoMethodError: private method `remove_const’ called for Object:Class
This is becoz, remove_const is a private method of Object class.
So the worry now is, how to invoke a private method.
We have two solutions:
- Object.send(:remove_const , : Person)
- Object.class_eval{remove_const : Person}
Now try, Person.new. # Curses, uninitialized constant Person
Voila, job done. Very rarely does an ERROR MESSAGE makes us feel happy. Well, kind of an article.
You can learn much about class_eval from this very good article by Khaled.
add attribute at runtime
I’m all excited abt this post.
Coz this is something i learnt just now, and consider it my best learning till now in RUBY.
Q: How to add attributes to a class at runtime?
A:
class Class
def new_attr (attr_name)
class_eval {attr_accessor :"#{attr_name}"}
end
end
class_eval : Evaluates a string or a block in the context of the receiver, here the class.
Explanation:
1) We defined a new method to the CLASS called new_attr, which is intended to add new attributes at runtime.
2) This method needs the name of the new attribute to be passed.
3) In the definition, it calls class_eval, which means “DO SOMETHING AT THE CLASS LEVEL”
4) What to do is specified in the block after the class_eval, which says : attr_accessor “#{attr_name}”, which will become attr_accessor :name when we call new_attr(‘name’).
5) We know that attr_accessor is rails syntactic sugar to create the a specific instance variable, and its set/get (read/write) methods. So here, when we call new_attr(‘name’), its creating a new instance variable (@name) and 2 new methods ‘name” and “name=”.
Try this;
class Person; end;
This creates a bare class Person.
p = Person.new
Try, p.name, it throws an error cursing “Undefined method name”.
Its fairly expected coz, Person has no attribute called name.
Now, say Person.new_attr(‘name’)
now try, p.name, it will not curse you.
To check, try Person.singleton_methods.include?(“name”)
This says TRUE.
Hope this article comes handy.
