Add support for read slave databases.

data/read_slave.py

@@ -0,0 +1,56 @@
+"""
+Adapted from:
+https://github.com/coleifer/peewee/blob/master/playhouse/read_slave.py
+
+Support for using a dedicated read-slave. The read database is specified as a
+Model.Meta option, and will be used for SELECT statements:
+
+
+master = PostgresqlDatabase('master')
+read_slave = PostgresqlDatabase('read_slave')
+
+class BaseModel(ReadSlaveModel):
+    class Meta:
+        database = master
+        read_slaves = [read_slave]  # This database will be used for SELECTs.
+
+
+# Now define your models as you would normally.
+class User(BaseModel):
+    username = CharField()
+
+# To force a SELECT on the master database, you can instantiate the SelectQuery
+# by hand:
+master_select = SelectQuery(User).where(...)
+"""
+from peewee import *
+
+
+class ReadSlaveModel(Model):
+  @classmethod
+  def _get_read_database(cls):
+    if (not getattr(cls._meta, 'read_slaves', None) or
+        cls._meta.database.transaction_depth() > 0):
+      return cls._meta.database
+    current_idx = getattr(cls, '_read_slave_idx', -1)
+    cls._read_slave_idx = (current_idx + 1) % len(cls._meta.read_slaves)
+    selected_read_slave = cls._meta.read_slaves[cls._read_slave_idx]
+
+    if isinstance(selected_read_slave, Proxy) and selected_read_slave.obj is None:
+      # It's possible the read slave was disabled by not initializing it
+      return cls._meta.database
+
+    return selected_read_slave
+
+  @classmethod
+  def select(cls, *args, **kwargs):
+    query = super(ReadSlaveModel, cls).select(*args, **kwargs)
+    query.database = cls._get_read_database()
+    return query
+
+  @classmethod
+  def raw(cls, *args, **kwargs):
+    query = super(ReadSlaveModel, cls).raw(*args, **kwargs)
+    if query._sql.lower().startswith('select'):
+        query.database = cls._get_read_database()
+    return query