Add ability to prevent writes to a database

This PR adds the ability to prevent writes to a database even if the
database user is able to write (ie the database is a primary and not a
replica).

This is useful for a few reasons: 1) when converting your database from
a single db to a primary/replica setup - you can fix all the writes on
reads early on, 2) when we implement automatic database switching or
when an app is manually switching connections this feature can be used
to ensure reads are reading and writes are writing. We want to make sure
we raise if we ever try to write in read mode, regardless of database
type and 3) for local development if you don't want to set up multiple
databases but do want to support rw/ro queries.

This should be used in conjunction with `connected_to` in write mode.
For example:

```
ActiveRecord::Base.connected_to(role: :writing) do
  Dog.connection.while_preventing_writes do
    Dog.create! # will raise because we're preventing writes
  end
end

ActiveRecord::Base.connected_to(role: :reading) do
  Dog.connection.while_preventing_writes do
    Dog.first # will not raise because we're not writing
  end
end
```

1 files changed, 16 insertions, 0 deletions

diff --git a/activerecord/CHANGELOG.md b/activerecord/CHANGELOG.md
index 5b07d63684..d9488bd0f0 100644
--- a/activerecord/CHANGELOG.md
+++ b/activerecord/CHANGELOG.md
@@ -1,3 +1,19 @@
+*   Add the ability to prevent writes to a database for the duration of a block.
+
+    Allows the application to prevent writes to a database. This can be useful when
+    you're building out multiple databases and want to make sure you're not sending
+    writes when you want a read.
+
+    If `while_preventing_writes` is called and the query is considered a write
+    query the database will raise an exception regardless of whether the database
+    user is able to write.
+
+    This is not meant to be a catch-all for write queries but rather a way to enforce
+    read-only queries without opening a second connection. One purpose of this is to
+    catch accidental writes, not all writes.
+
+    *Eileen M. Uchitelle*
+
 *   Allow aliased attributes to be used in `#update_columns` and `#update`.
 
     *Gannon McGibbon*