synapse-product/synapse/storage/_base.py

# -*- coding: utf-8 -*-
# Copyright 2014, 2015 OpenMarket Ltd
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#     http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.
import logging

from synapse.api.errors import StoreError
from synapse.util.logutils import log_function
from synapse.util.logcontext import preserve_context_over_fn, LoggingContext
from synapse.util.caches.dictionary_cache import DictionaryCache
from synapse.util.caches.descriptors import Cache
import synapse.metrics

from util.id_generators import IdGenerator, StreamIdGenerator

from twisted.internet import defer

from collections import namedtuple

import sys
import time
import threading


logger = logging.getLogger(__name__)

sql_logger = logging.getLogger("synapse.storage.SQL")
transaction_logger = logging.getLogger("synapse.storage.txn")
perf_logger = logging.getLogger("synapse.storage.TIME")


metrics = synapse.metrics.get_metrics_for("synapse.storage")

sql_scheduling_timer = metrics.register_distribution("schedule_time")

sql_query_timer = metrics.register_distribution("query_time", labels=["verb"])
sql_txn_timer = metrics.register_distribution("transaction_time", labels=["desc"])


class LoggingTransaction(object):
    """An object that almost-transparently proxies for the 'txn' object
    passed to the constructor. Adds logging and metrics to the .execute()
    method."""
    __slots__ = ["txn", "name", "database_engine", "after_callbacks"]

    def __init__(self, txn, name, database_engine, after_callbacks):
        object.__setattr__(self, "txn", txn)
        object.__setattr__(self, "name", name)
        object.__setattr__(self, "database_engine", database_engine)
        object.__setattr__(self, "after_callbacks", after_callbacks)

    def call_after(self, callback, *args):
        """Call the given callback on the main twisted thread after the
        transaction has finished. Used to invalidate the caches on the
        correct thread.
        """
        self.after_callbacks.append((callback, args))

    def __getattr__(self, name):
        return getattr(self.txn, name)

    def __setattr__(self, name, value):
        setattr(self.txn, name, value)

    def execute(self, sql, *args):
        self._do_execute(self.txn.execute, sql, *args)

    def executemany(self, sql, *args):
        self._do_execute(self.txn.executemany, sql, *args)

    def _do_execute(self, func, sql, *args):
        # TODO(paul): Maybe use 'info' and 'debug' for values?
        sql_logger.debug("[SQL] {%s} %s", self.name, sql)

        sql = self.database_engine.convert_param_style(sql)

        if args:
            try:
                sql_logger.debug(
                    "[SQL values] {%s} %r",
                    self.name, args[0]
                )
            except:
                # Don't let logging failures stop SQL from working
                pass

        start = time.time() * 1000

        try:
            return func(
                sql, *args
            )
        except Exception as e:
            logger.debug("[SQL FAIL] {%s} %s", self.name, e)
            raise
        finally:
            msecs = (time.time() * 1000) - start
            sql_logger.debug("[SQL time] {%s} %f", self.name, msecs)
            sql_query_timer.inc_by(msecs, sql.split()[0])


class PerformanceCounters(object):
    def __init__(self):
        self.current_counters = {}
        self.previous_counters = {}

    def update(self, key, start_time, end_time=None):
        if end_time is None:
            end_time = time.time() * 1000
        duration = end_time - start_time
        count, cum_time = self.current_counters.get(key, (0, 0))
        count += 1
        cum_time += duration
        self.current_counters[key] = (count, cum_time)
        return end_time

    def interval(self, interval_duration, limit=3):
        counters = []
        for name, (count, cum_time) in self.current_counters.items():
            prev_count, prev_time = self.previous_counters.get(name, (0, 0))
            counters.append((
                (cum_time - prev_time) / interval_duration,
                count - prev_count,
                name
            ))

        self.previous_counters = dict(self.current_counters)

        counters.sort(reverse=True)

        top_n_counters = ", ".join(
            "%s(%d): %.3f%%" % (name, count, 100 * ratio)
            for ratio, count, name in counters[:limit]
        )

        return top_n_counters


class SQLBaseStore(object):
    _TXN_ID = 0

    def __init__(self, hs):
        self.hs = hs
        self._db_pool = hs.get_db_pool()
        self._clock = hs.get_clock()

        self._previous_txn_total_time = 0
        self._current_txn_total_time = 0
        self._previous_loop_ts = 0

        # TODO(paul): These can eventually be removed once the metrics code
        #   is running in mainline, and we have some nice monitoring frontends
        #   to watch it
        self._txn_perf_counters = PerformanceCounters()
        self._get_event_counters = PerformanceCounters()

        self._get_event_cache = Cache("*getEvent*", keylen=3, lru=True,
                                      max_entries=hs.config.event_cache_size)

        self._state_group_cache = DictionaryCache("*stateGroupCache*", 2000)

        self._event_fetch_lock = threading.Condition()
        self._event_fetch_list = []
        self._event_fetch_ongoing = 0

        self._pending_ds = []

        self.database_engine = hs.database_engine

        self._stream_id_gen = StreamIdGenerator("events", "stream_ordering")
        self._transaction_id_gen = IdGenerator("sent_transactions", "id", self)
        self._state_groups_id_gen = IdGenerator("state_groups", "id", self)
        self._access_tokens_id_gen = IdGenerator("access_tokens", "id", self)
        self._pushers_id_gen = IdGenerator("pushers", "id", self)
        self._push_rule_id_gen = IdGenerator("push_rules", "id", self)
        self._push_rules_enable_id_gen = IdGenerator("push_rules_enable", "id", self)
        self._receipts_id_gen = StreamIdGenerator("receipts_linearized", "stream_id")

    def start_profiling(self):
        self._previous_loop_ts = self._clock.time_msec()

        def loop():
            curr = self._current_txn_total_time
            prev = self._previous_txn_total_time
            self._previous_txn_total_time = curr

            time_now = self._clock.time_msec()
            time_then = self._previous_loop_ts
            self._previous_loop_ts = time_now

            ratio = (curr - prev)/(time_now - time_then)

            top_three_counters = self._txn_perf_counters.interval(
                time_now - time_then, limit=3
            )

            top_3_event_counters = self._get_event_counters.interval(
                time_now - time_then, limit=3
            )

            perf_logger.info(
                "Total database time: %.3f%% {%s} {%s}",
                ratio * 100, top_three_counters, top_3_event_counters
            )

        self._clock.looping_call(loop, 10000)

    def _new_transaction(self, conn, desc, after_callbacks, func, *args, **kwargs):
        start = time.time() * 1000
        txn_id = self._TXN_ID

        # We don't really need these to be unique, so lets stop it from
        # growing really large.
        self._TXN_ID = (self._TXN_ID + 1) % (sys.maxint - 1)

        name = "%s-%x" % (desc, txn_id, )

        transaction_logger.debug("[TXN START] {%s}", name)

        try:
            i = 0
            N = 5
            while True:
                try:
                    txn = conn.cursor()
                    txn = LoggingTransaction(
                        txn, name, self.database_engine, after_callbacks
                    )
                    r = func(txn, *args, **kwargs)
                    conn.commit()
                    return r
                except self.database_engine.module.OperationalError as e:
                    # This can happen if the database disappears mid
                    # transaction.
                    logger.warn(
                        "[TXN OPERROR] {%s} %s %d/%d",
                        name, e, i, N
                    )
                    if i < N:
                        i += 1
                        try:
                            conn.rollback()
                        except self.database_engine.module.Error as e1:
                            logger.warn(
                                "[TXN EROLL] {%s} %s",
                                name, e1,
                            )
                        continue
                    raise
                except self.database_engine.module.DatabaseError as e:
                    if self.database_engine.is_deadlock(e):
                        logger.warn("[TXN DEADLOCK] {%s} %d/%d", name, i, N)
                        if i < N:
                            i += 1
                            try:
                                conn.rollback()
                            except self.database_engine.module.Error as e1:
                                logger.warn(
                                    "[TXN EROLL] {%s} %s",
                                    name, e1,
                                )
                            continue
                    raise
        except Exception as e:
            logger.debug("[TXN FAIL] {%s} %s", name, e)
            raise
        finally:
            end = time.time() * 1000
            duration = end - start

            transaction_logger.debug("[TXN END] {%s} %f", name, duration)

            self._current_txn_total_time += duration
            self._txn_perf_counters.update(desc, start, end)
            sql_txn_timer.inc_by(duration, desc)

    @defer.inlineCallbacks
    def runInteraction(self, desc, func, *args, **kwargs):
        """Wraps the .runInteraction() method on the underlying db_pool."""
        current_context = LoggingContext.current_context()

        start_time = time.time() * 1000

        after_callbacks = []

        def inner_func(conn, *args, **kwargs):
            with LoggingContext("runInteraction") as context:
                sql_scheduling_timer.inc_by(time.time() * 1000 - start_time)

                if self.database_engine.is_connection_closed(conn):
                    logger.debug("Reconnecting closed database connection")
                    conn.reconnect()

                current_context.copy_to(context)
                return self._new_transaction(
                    conn, desc, after_callbacks, func, *args, **kwargs
                )

        result = yield preserve_context_over_fn(
            self._db_pool.runWithConnection,
            inner_func, *args, **kwargs
        )

        for after_callback, after_args in after_callbacks:
            after_callback(*after_args)
        defer.returnValue(result)

    @defer.inlineCallbacks
    def runWithConnection(self, func, *args, **kwargs):
        """Wraps the .runInteraction() method on the underlying db_pool."""
        current_context = LoggingContext.current_context()

        start_time = time.time() * 1000

        def inner_func(conn, *args, **kwargs):
            with LoggingContext("runWithConnection") as context:
                sql_scheduling_timer.inc_by(time.time() * 1000 - start_time)

                if self.database_engine.is_connection_closed(conn):
                    logger.debug("Reconnecting closed database connection")
                    conn.reconnect()

                current_context.copy_to(context)

                return func(conn, *args, **kwargs)

        result = yield preserve_context_over_fn(
            self._db_pool.runWithConnection,
            inner_func, *args, **kwargs
        )

        defer.returnValue(result)

    def cursor_to_dict(self, cursor):
        """Converts a SQL cursor into an list of dicts.

        Args:
            cursor : The DBAPI cursor which has executed a query.
        Returns:
            A list of dicts where the key is the column header.
        """
        col_headers = list(column[0] for column in cursor.description)
        results = list(
            dict(zip(col_headers, row)) for row in cursor.fetchall()
        )
        return results

    def _execute(self, desc, decoder, query, *args):
        """Runs a single query for a result set.

        Args:
            decoder - The function which can resolve the cursor results to
                something meaningful.
            query - The query string to execute
            *args - Query args.
        Returns:
            The result of decoder(results)
        """
        def interaction(txn):
            txn.execute(query, args)
            if decoder:
                return decoder(txn)
            else:
                return txn.fetchall()

        return self.runInteraction(desc, interaction)

    def _execute_and_decode(self, desc, query, *args):
        return self._execute(desc, self.cursor_to_dict, query, *args)

    # "Simple" SQL API methods that operate on a single table with no JOINs,
    # no complex WHERE clauses, just a dict of values for columns.

    @defer.inlineCallbacks
    def _simple_insert(self, table, values, or_ignore=False,
                       desc="_simple_insert"):
        """Executes an INSERT query on the named table.

        Args:
            table : string giving the table name
            values : dict of new column names and values for them
        """
        try:
            yield self.runInteraction(
                desc,
                self._simple_insert_txn, table, values,
            )
        except self.database_engine.module.IntegrityError:
            # We have to do or_ignore flag at this layer, since we can't reuse
            # a cursor after we receive an error from the db.
            if not or_ignore:
                raise

    @log_function
    def _simple_insert_txn(self, txn, table, values):
        keys, vals = zip(*values.items())

        sql = "INSERT INTO %s (%s) VALUES(%s)" % (
            table,
            ", ".join(k for k in keys),
            ", ".join("?" for _ in keys)
        )

        txn.execute(sql, vals)

    def _simple_insert_many_txn(self, txn, table, values):
        if not values:
            return

        # This is a *slight* abomination to get a list of tuples of key names
        # and a list of tuples of value names.
        #
        # i.e. [{"a": 1, "b": 2}, {"c": 3, "d": 4}]
        #         => [("a", "b",), ("c", "d",)] and [(1, 2,), (3, 4,)]
        #
        # The sort is to ensure that we don't rely on dictionary iteration
        # order.
        keys, vals = zip(*[
            zip(
                *(sorted(i.items(), key=lambda kv: kv[0]))
            )
            for i in values
            if i
        ])

        for k in keys:
            if k != keys[0]:
                raise RuntimeError(
                    "All items must have the same keys"
                )

        sql = "INSERT INTO %s (%s) VALUES(%s)" % (
            table,
            ", ".join(k for k in keys[0]),
            ", ".join("?" for _ in keys[0])
        )

        txn.executemany(sql, vals)

    def _simple_upsert(self, table, keyvalues, values,
                       insertion_values={}, desc="_simple_upsert", lock=True):
        """
        Args:
            table (str): The table to upsert into
            keyvalues (dict): The unique key tables and their new values
            values (dict): The nonunique columns and their new values
            insertion_values (dict): key/values to use when inserting
        Returns: A deferred
        """
        return self.runInteraction(
            desc,
            self._simple_upsert_txn, table, keyvalues, values, insertion_values,
            lock
        )

    def _simple_upsert_txn(self, txn, table, keyvalues, values, insertion_values={},
                           lock=True):
        # We need to lock the table :(, unless we're *really* careful
        if lock:
            self.database_engine.lock_table(txn, table)

        # Try to update
        sql = "UPDATE %s SET %s WHERE %s" % (
            table,
            ", ".join("%s = ?" % (k,) for k in values),
            " AND ".join("%s = ?" % (k,) for k in keyvalues)
        )
        sqlargs = values.values() + keyvalues.values()
        logger.debug(
            "[SQL] %s Args=%s",
            sql, sqlargs,
        )

        txn.execute(sql, sqlargs)
        if txn.rowcount == 0:
            # We didn't update and rows so insert a new one
            allvalues = {}
            allvalues.update(keyvalues)
            allvalues.update(values)
            allvalues.update(insertion_values)

            sql = "INSERT INTO %s (%s) VALUES (%s)" % (
                table,
                ", ".join(k for k in allvalues),
                ", ".join("?" for _ in allvalues)
            )
            logger.debug(
                "[SQL] %s Args=%s",
                sql, keyvalues.values(),
            )
            txn.execute(sql, allvalues.values())

    def _simple_select_one(self, table, keyvalues, retcols,
                           allow_none=False, desc="_simple_select_one"):
        """Executes a SELECT query on the named table, which is expected to
        return a single row, returning a single column from it.

        Args:
            table : string giving the table name
            keyvalues : dict of column names and values to select the row with
            retcols : list of strings giving the names of the columns to return

            allow_none : If true, return None instead of failing if the SELECT
              statement returns no rows
        """
        return self.runInteraction(
            desc,
            self._simple_select_one_txn,
            table, keyvalues, retcols, allow_none,
        )

    def _simple_select_one_onecol(self, table, keyvalues, retcol,
                                  allow_none=False,
                                  desc="_simple_select_one_onecol"):
        """Executes a SELECT query on the named table, which is expected to
        return a single row, returning a single column from it."

        Args:
            table : string giving the table name
            keyvalues : dict of column names and values to select the row with
            retcol : string giving the name of the column to return
        """
        return self.runInteraction(
            desc,
            self._simple_select_one_onecol_txn,
            table, keyvalues, retcol, allow_none=allow_none,
        )

    def _simple_select_one_onecol_txn(self, txn, table, keyvalues, retcol,
                                      allow_none=False):
        ret = self._simple_select_onecol_txn(
            txn,
            table=table,
            keyvalues=keyvalues,
            retcol=retcol,
        )

        if ret:
            return ret[0]
        else:
            if allow_none:
                return None
            else:
                raise StoreError(404, "No row found")

    def _simple_select_onecol_txn(self, txn, table, keyvalues, retcol):
        sql = (
            "SELECT %(retcol)s FROM %(table)s WHERE %(where)s"
        ) % {
            "retcol": retcol,
            "table": table,
            "where": " AND ".join("%s = ?" % k for k in keyvalues.keys()),
        }

        txn.execute(sql, keyvalues.values())

        return [r[0] for r in txn.fetchall()]

    def _simple_select_onecol(self, table, keyvalues, retcol,
                              desc="_simple_select_onecol"):
        """Executes a SELECT query on the named table, which returns a list
        comprising of the values of the named column from the selected rows.

        Args:
            table (str): table name
            keyvalues (dict): column names and values to select the rows with
            retcol (str): column whos value we wish to retrieve.

        Returns:
            Deferred: Results in a list
        """
        return self.runInteraction(
            desc,
            self._simple_select_onecol_txn,
            table, keyvalues, retcol
        )

    def _simple_select_list(self, table, keyvalues, retcols,
                            desc="_simple_select_list"):
        """Executes a SELECT query on the named table, which may return zero or
        more rows, returning the result as a list of dicts.

        Args:
            table : string giving the table name
            keyvalues : dict of column names and values to select the rows with,
            or None to not apply a WHERE clause.
            retcols : list of strings giving the names of the columns to return
        """
        return self.runInteraction(
            desc,
            self._simple_select_list_txn,
            table, keyvalues, retcols
        )

    def _simple_select_list_txn(self, txn, table, keyvalues, retcols):
        """Executes a SELECT query on the named table, which may return zero or
        more rows, returning the result as a list of dicts.

        Args:
            txn : Transaction object
            table : string giving the table name
            keyvalues : dict of column names and values to select the rows with
            retcols : list of strings giving the names of the columns to return
        """
        if keyvalues:
            sql = "SELECT %s FROM %s WHERE %s" % (
                ", ".join(retcols),
                table,
                " AND ".join("%s = ?" % (k, ) for k in keyvalues)
            )
            txn.execute(sql, keyvalues.values())
        else:
            sql = "SELECT %s FROM %s" % (
                ", ".join(retcols),
                table
            )
            txn.execute(sql)

        return self.cursor_to_dict(txn)

    def _simple_update_one(self, table, keyvalues, updatevalues,
                           desc="_simple_update_one"):
        """Executes an UPDATE query on the named table, setting new values for
        columns in a row matching the key values.

        Args:
            table : string giving the table name
            keyvalues : dict of column names and values to select the row with
            updatevalues : dict giving column names and values to update
            retcols : optional list of column names to return

        If present, retcols gives a list of column names on which to perform
        a SELECT statement *before* performing the UPDATE statement. The values
        of these will be returned in a dict.

        These are performed within the same transaction, allowing an atomic
        get-and-set.  This can be used to implement compare-and-set by putting
        the update column in the 'keyvalues' dict as well.
        """
        return self.runInteraction(
            desc,
            self._simple_update_one_txn,
            table, keyvalues, updatevalues,
        )

    def _simple_update_one_txn(self, txn, table, keyvalues, updatevalues):
        update_sql = "UPDATE %s SET %s WHERE %s" % (
            table,
            ", ".join("%s = ?" % (k,) for k in updatevalues),
            " AND ".join("%s = ?" % (k,) for k in keyvalues)
        )

        txn.execute(
            update_sql,
            updatevalues.values() + keyvalues.values()
        )

        if txn.rowcount == 0:
            raise StoreError(404, "No row found")
        if txn.rowcount > 1:
            raise StoreError(500, "More than one row matched")

    def _simple_select_one_txn(self, txn, table, keyvalues, retcols,
                               allow_none=False):
        select_sql = "SELECT %s FROM %s WHERE %s" % (
            ", ".join(retcols),
            table,
            " AND ".join("%s = ?" % (k,) for k in keyvalues)
        )

        txn.execute(select_sql, keyvalues.values())

        row = txn.fetchone()
        if not row:
            if allow_none:
                return None
            raise StoreError(404, "No row found")
        if txn.rowcount > 1:
            raise StoreError(500, "More than one row matched")

        return dict(zip(retcols, row))

    def _simple_selectupdate_one(self, table, keyvalues, updatevalues=None,
                                 retcols=None, allow_none=False,
                                 desc="_simple_selectupdate_one"):
        """ Combined SELECT then UPDATE."""
        def func(txn):
            ret = None
            if retcols:
                ret = self._simple_select_one_txn(
                    txn,
                    table=table,
                    keyvalues=keyvalues,
                    retcols=retcols,
                    allow_none=allow_none,
                )

            if updatevalues:
                self._simple_update_one_txn(
                    txn,
                    table=table,
                    keyvalues=keyvalues,
                    updatevalues=updatevalues,
                )

                # if txn.rowcount == 0:
                #     raise StoreError(404, "No row found")
                if txn.rowcount > 1:
                    raise StoreError(500, "More than one row matched")

            return ret
        return self.runInteraction(desc, func)

    def _simple_delete_one(self, table, keyvalues, desc="_simple_delete_one"):
        """Executes a DELETE query on the named table, expecting to delete a
        single row.

        Args:
            table : string giving the table name
            keyvalues : dict of column names and values to select the row with
        """
        sql = "DELETE FROM %s WHERE %s" % (
            table,
            " AND ".join("%s = ?" % (k, ) for k in keyvalues)
        )

        def func(txn):
            txn.execute(sql, keyvalues.values())
            if txn.rowcount == 0:
                raise StoreError(404, "No row found")
            if txn.rowcount > 1:
                raise StoreError(500, "more than one row matched")
        return self.runInteraction(desc, func)

    def _simple_delete(self, table, keyvalues, desc="_simple_delete"):
        """Executes a DELETE query on the named table.

        Args:
            table : string giving the table name
            keyvalues : dict of column names and values to select the row with
        """

        return self.runInteraction(desc, self._simple_delete_txn)

    def _simple_delete_txn(self, txn, table, keyvalues):
        sql = "DELETE FROM %s WHERE %s" % (
            table,
            " AND ".join("%s = ?" % (k, ) for k in keyvalues)
        )

        return txn.execute(sql, keyvalues.values())

    def _simple_max_id(self, table):
        """Executes a SELECT query on the named table, expecting to return the
        max value for the column "id".

        Args:
            table : string giving the table name
        """
        sql = "SELECT MAX(id) AS id FROM %s" % table

        def func(txn):
            txn.execute(sql)
            max_id = self.cursor_to_dict(txn)[0]["id"]
            if max_id is None:
                return 0
            return max_id

        return self.runInteraction("_simple_max_id", func)

    def get_next_stream_id(self):
        with self._next_stream_id_lock:
            i = self._next_stream_id
            self._next_stream_id += 1
            return i


class _RollbackButIsFineException(Exception):
    """ This exception is used to rollback a transaction without implying
    something went wrong.
    """
    pass


class Table(object):
    """ A base class used to store information about a particular table.
    """

    table_name = None
    """ str: The name of the table """

    fields = None
    """ list: The field names """

    EntryType = None
    """ Type: A tuple type used to decode the results """

    _select_where_clause = "SELECT %s FROM %s WHERE %s"
    _select_clause = "SELECT %s FROM %s"
    _insert_clause = "REPLACE INTO %s (%s) VALUES (%s)"

    @classmethod
    def select_statement(cls, where_clause=None):
        """
        Args:
            where_clause (str): The WHERE clause to use.

        Returns:
            str: An SQL statement to select rows from the table with the given
            WHERE clause.
        """
        if where_clause:
            return cls._select_where_clause % (
                ", ".join(cls.fields),
                cls.table_name,
                where_clause
            )
        else:
            return cls._select_clause % (
                ", ".join(cls.fields),
                cls.table_name,
            )

    @classmethod
    def insert_statement(cls):
        return cls._insert_clause % (
            cls.table_name,
            ", ".join(cls.fields),
            ", ".join(["?"] * len(cls.fields)),
        )

    @classmethod
    def decode_single_result(cls, results):
        """ Given an iterable of tuples, return a single instance of
            `EntryType` or None if the iterable is empty
        Args:
            results (list): The results list to convert to `EntryType`
        Returns:
            EntryType: An instance of `EntryType`
        """
        results = list(results)
        if results:
            return cls.EntryType(*results[0])
        else:
            return None

    @classmethod
    def decode_results(cls, results):
        """ Given an iterable of tuples, return a list of `EntryType`
        Args:
            results (list): The results list to convert to `EntryType`

        Returns:
            list: A list of `EntryType`
        """
        return [cls.EntryType(*row) for row in results]

    @classmethod
    def get_fields_string(cls, prefix=None):
        if prefix:
            to_join = ("%s.%s" % (prefix, f) for f in cls.fields)
        else:
            to_join = cls.fields

        return ", ".join(to_join)


class JoinHelper(object):
    """ Used to help do joins on tables by looking at the tables' fields and
    creating a list of unique fields to use with SELECTs and a namedtuple
    to dump the results into.

    Attributes:
        tables (list): List of `Table` classes
        EntryType (type)
    """

    def __init__(self, *tables):
        self.tables = tables

        res = []
        for table in self.tables:
            res += [f for f in table.fields if f not in res]

        self.EntryType = namedtuple("JoinHelperEntry", res)

    def get_fields(self, **prefixes):
        """Get a string representing a list of fields for use in SELECT
        statements with the given prefixes applied to each.

        For example::

            JoinHelper(PdusTable, StateTable).get_fields(
                PdusTable="pdus",
                StateTable="state"
            )
        """
        res = []
        for field in self.EntryType._fields:
            for table in self.tables:
                if field in table.fields:
                    res.append("%s.%s" % (prefixes[table.__name__], field))
                    break

        return ", ".join(res)

    def decode_results(self, rows):
        return [self.EntryType(*row) for row in rows]