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Ruby on Rails - reverse add_column, add_index migration


As per my understanding you want to rollback the migration. If so then you can use the below syntax for that

rake db:rollback STEP=n

       or


rake db:migrate:down
VERSION=<version_number_of_migration>

which rollsback the particular migration


Categories : Ruby On Rails

Related to : Ruby on Rails - reverse add_column, add_index migration
Editing rails migration table doesn't affect tables generated
This is exactly what you want. It creates the same column, while adding an index as well. add_reference creates the column with the name you specified, which in your case is the same as the original migration. If you run SHOW INDEXES on the table, you should see the newly created index as well.

Categories : Mysql
R: fill a new column in a data frame with a value by matching variables in reverse
Try merge. The by.x and by.y arguments specify columns to be matched, and here the order of matching columns is reversed in by.y: merge(x = df, y = df, by.x = c("x", "y"), by.y = c("y", "x")) # x y value_xy.x value_xy.y # 1 A B 7 3 # 2 A C 2 2 # 3 B A 3 7 # 4 B C 6 1 # 5 C A 2 2 # 6 C B 1

Categories : R
Update table by column index not by column name
As per the comments you can do it with dynamic SQL (not recommended) due to security implications or the below. Update Products Set ID = IF(colparameter=1, valueparam, ID), name = IF(colparameter=2, valueparam, name), price = IF(colparameter=3, valueparam, price) where ID = 5 Even this I wouldn't recommend doing.

Categories : Mysql
Rails index uniqueness with specific value
I don't believe you can have conditional uniqueness constraints at the database layer (via migrations). You can add this as a conditional validation at the AR layer though which should be sufficient for your purposes (though it should be noted this can introduce some race conditions). ie. validates [:col1, :col2], uniqueness: true, if: ":col3 == true" Hope that helps.

Categories : Ruby On Rails
Convert a column to an index in pandas
You can simply use the set_index method for this: df.set_index('Number') This take the column out of the DataFrame and sets it as the DataFrame's index. The method also allows you to quickly set multiple columns as indexes or check whether the new index contains duplicates.

Categories : Python
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