Ruby-on-rails Rails 迁移：检查存在并继续？
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Rails Migrations: Check Existence and Keep Going?
提问by Dan Rosenstark
I was doing this kind of thing in my migrations:
我在迁移中做这样的事情：
add_column :statuses, :hold_reason, :string rescue puts "column already added"
but it turns out that, while this works for SQLite, it does not work for PostgreSQL. It seems like if the add_column blows up, even if the Exception is caught,the transaction is dead and so the Migration can't do any additional work.
但事实证明，虽然这适用于 SQLite，但它不适用于 PostgreSQL。看起来如果 add_column 爆炸了，即使捕获到异常，事务也死了，因此迁移无法做任何额外的工作。
Is there any non-DB sepecificways to check if a column or table already exist? Failing that, is there any way to get my rescue block to really work?
是否有任何非数据库特定的方法来检查列或表是否已存在？如果做不到这一点，有什么方法可以让我的救援块真正起作用吗？
回答by Tobias Cohen
As of Rails 3.0 and later, you can use column_exists?to check for the existance of a column.
从 Rails 3.0 及更高版本开始，您可以使用column_exists?来检查列是否存在。
unless column_exists? :statuses, :hold_reason
add_column :statuses, :hold_reason, :string
end
There's also a table_exists?function, which goes as far back as Rails 2.1.
还有一个table_exists?函数，它可以追溯到 Rails 2.1。
回答by SG 86
Or even shorter
甚至更短
add_column :statuses, :hold_reason, :string unless column_exists? :statuses, :hold_reason
回答by JellicleCat
For Rails 2.X, you can check the existence of columns with the following:
对于Rails 2.X，您可以使用以下内容检查列是否存在：
columns("[table-name]").index {|col| col.name == "[column-name]"}
If it returns nil, no such column exists. If it returns a Fixnum, then the column does exist. Naturally, you can put more selective parameters between the {...}if you want to identify a column by more than just its name, for example:
如果它返回 nil，则不存在这样的列。如果它返回一个 Fixnum，那么该列确实存在。当然，{...}如果您想通过不仅仅是名称来标识列，则可以在 之间放置更多选择性参数，例如：
{ |col| col.name == "foo" and col.sql_type == "tinyint(1)" and col.primary == nil }
(this answer first posted on How to write conditional migrations in rails?)
（这个答案首先发布在如何在 Rails 中编写条件迁移？）
回答by Denis Neverov
add_column :statuses, :hold_reason, :string unless Status.column_names.include?("hold_reason")
add_column :statuses, :hold_reason, :string unless Status.column_names.include?("hold_reason")
