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Ruby on Rails: How do I add a not null constraint to an existing column using a migration?
In my Rails (3.2) app, I have a bunch of tables in my database but I forgot to add a few not null constraints. I've googled around but I can't find how to write a migration which adds not null to an existing column.
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mmap() vs. reading blocks
...r random access, especially if your access patterns are sparse and unpredictable.
Memory maps allow you to keep using pages from the cache until you are done. This means that if you use a file heavily for a long period of time, then close it and reopen it, the pages will still be cached. With read...
boost Composite keys - C/C++ - 清泛网 - 专注C/C++及内核技术
...lational databases, composite keys depend on two or more fields of a given table The analogous concept in Boost MultiIndex is Composite keys
In relational databases, composite keys depend on two or more fields of a given table. The analogous concept in Boost.MultiIndex is modeled by means of com...
Omit rows containing specific column of NA
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It is possible to use na.omit for data.table:
na.omit(data, cols = c("x", "z"))
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SQLite UPSERT / UPDATE OR INSERT
... there are two ways to accomplish this, depending on the structure of your table and if you have foreign keys restrictions activated to maintain integrity. I'd like to share this in a clean format to save some time to the people that may be in my situation.
Option 1: You can afford deleting the row...
CMake使用教程 - C/C++ - 清泛网 - 专注C/C++及内核技术
...ab/testngpp/cmake-2.8.1/Tests/Tutorial/Step1/build3>make
Linking CXX executable Tutorial.exe
[100%] Built target Tutorial
可以运行一下Turorial.exe：
D:/Projects/Lab/testngpp/cmake-2.8.1/Tests/Tutorial/Step1/build3>Tutorial.exe
Tutorial.exe Version 1.0
Usage: Tutorial.exe number
D:/Pr...
Extracting hours from a DateTime (SQL Server 2005)
...dd(hour,12,time_received)
else time_received
END
from table
works
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创建增量同步Oracle物化视图问题 - 数据库(内核) - 清泛网 - 专注C/C++及内核技术
...个表上建立物化视图日志如“CREATE MATERIALIZED VIEW LOG ON T_tablename;”，再到数据库B上创建数据库链接和快速刷新的物化视图如“create materialized view mv_tablename refresh fast on demand start with sysdate next sysdate+1/288 as select * from T_tablename@dblink_...
Is std::unique_ptr required to know the full definition of T?
... compiler will tell you.
However, in case it is helpful to you, here is a table which documents several members of shared_ptr and unique_ptr with respect to completeness requirements. If the member requires a complete type, then entry has a "C", otherwise the table entry is filled with "I".
Comple...
NSDefaultRunLoopMode vs NSRunLoopCommonModes
... in different modes. A "run loop mode" is a convention used by the OS to establish some rules for when to deliver certain events or collect them to be delivered later.
Usually all run loops are set to the "default mode" which establishes a default way to manage input events. For example: as soon as...