DB2 on Rails provides the developer with many ways to retrieve information about errors which have occurred. Rails developers can take a look at the logs within the log folder in order to read the SQL errors returned by the datasever. But what about simple Ruby scripts? Let’s try to connect to DB2 with the wrong user credentials:
conn = IBM_DB::connect("mydb","myuser","my_wrong_pass")
#=> false
false. That doesn’t really help us too much. We know that something went wrong, but we don’t exactly know what. Luckily for us, the IBM_DB driver gives us all the tools that we need to properly troubleshoot problems. The example above can be rewritten in this way:
if conn = IBM_DB::connect("mydb","myuser","my_wrong_pass")
# do something interesting
else
# conn is false
raise IBM_DB::conn_errormsg
end
#=> RuntimeError: [IBM][CLI Driver] SQL30082N Security processing failed with reason
“24″ (”USERNAME AND/OR PASSWORD INVALID”). SQLSTATE=08001 SQLCODE=-30082
Okay, that should have us covered when it comes to failed attempts to connect to the database, but what about failed queries? Analogously, the IBM_DB driver provides us with the IBM_DB::stmt_errormsg method:
if stmt = IBM_DB::exec(conn, "SELECT * FROM WRONG_TABLE)
# process the results
else
# stmt is false
raise IBM_DB::stmt_errormsg
end
#=> RuntimeError: => [IBM][CLI Driver][DB2/NT] SQL0204N " DB2ADMIN.WRONG_TABLE" is
an undefined name. SQLSTATE=42704 SQLCODE=-204
DB2 error messages are usually easy to understand and with a bit of help from the DB2 Information Center you should be able to get out of trouble most of the time.
DB2 CLI Tracing
Despite the helpful error messages, there are situations in which troubleshooting can be hard because everything seems to be in the right place but your application is still acting up for some reason. The ultimate tool when it comes to troubleshooting for DB2 and Ruby/Rails application problems is to enable DB2 CLI (Call Level Interface) tracing. The CLI trace captures information about applications that access the DB2 CLI driver. Tracing gives you the ability to analyze low level calls to the C driver API with details on the input and output, to and from the database. The resulting trace is not particularly easy to understand for a DB2 novice, but it can offer a microscopic view which is invaluable for understanding problems that are hidden by the abstractions of higher level APIs, and allow you to see to a certain degree what’s happening under the hood.
The two free PDFs, Call Level Interface Guide and Reference, Volume 1 and Volume 2 are the best references if you need to look up calls in your CLI trace files. Instructions on how to enable CLI level tracing for DB2 Express-C LUW (Linux/Unix/Windows) can be found here.
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October 19 2007 | How-to and Tips&Tricks | 5 Comments »
With an increasing number of Rails developers adopting DB2 as their database of choice, and the welcoming approach towards suggestions and requests on our end, it comes as no surprise that the feedback is rolling in faster than ever. We are very pleased about this, and in this short article I’d love to address two of the most requested features.
rake db:test:purge
Running rake db:test:purge generates the following error:
rake aborted!
Task not supported by 'ibm_db'
db:test:purge is a rake task defined in the Rails gem within the file rails-1.2.3/lib/tasks/databases.rake. This file takes care of, amongst other things, defining the specific behavior for each database adapter known to rails, when the user requests a database purge. It is essentially a big case statement with specific ruby code which drops all the user objects from the supported database.
ibm_db is currently shipped independently from Rails, therefore it is missing from that case statement, and as a result the task will appear to not be supported by ibm_db.
If you are using the ibm_db adapter for DB2, you don’t want to miss the opportunity to use rake db:test:purge and take advantage of other tasks that rely on this in order to work. What you can do is, manually insert a snippet of code for the ibm_db case in databases.rake.
An easy and straightforward implementation of this would be dropping the database and recreating it from scratch. I’m not allowed into the other adapters implementations, but I assume this could be what some of the other adapters may be doing. I believe this is not the right thing to do with DB2 though. In fact, this would have two main drawbacks. Firstly, creating a database every time, implies that the task becomes quite slow, because the creation of a database in DB2 is a “magical process” that can take up to a minute (for good reason, and that minute can save you lots of money in the years to come as you use the database). Secondly, the database that you have created in the first place may have many options and parameters configured, and collecting them all and reapplying them may not be the easiest or the smartest thing to do.
A different approach would be to handle this the right way, by dropping all the user schemas and the objects contained within the database. The code needs to be placed within the case statement we mentioned above:
desc “Empty the test database“
task :purge => :environment do
abcs = ActiveRecord::Base.configurations
case abcs[”test“][”adapter“]
end
end
With the current (Rails 1.2.3) databases.rake you can practically just copy and paste the following code at line 145 in the file:
when “ibm_db“
ActiveRecord::Base.establish_connection(:test)
conn = ActiveRecord::Base.connection.connection
begin
ActiveRecord::Base.connection.execute(”CREATE TABLESPACE SYSTOOLSPACE“)
systool_existing = false
rescue
systool_existing = true
end
user_schemas_sql = “SELECT SCHEMANAME FROM SYSCAT.SCHEMATA WHERE DEFINER <> ‘SYSIBM’ AND
SCHEMANAME NOT IN (’NULLID’, ‘ERRORSCHEMA’, ‘SYSTOOLS’)“
schemas = ActiveRecord::Base.connection.select_all(user_schemas_sql)
unless schemas.empty?
errortabschema = ‘ERRORSCHEMA‘
errortab = ‘ERRORTABLE‘
schemas.each do |schema|
schema_name = schema[”schemaname“].strip.upcase
sql = “CALL SYSPROC.ADMIN_DROP_SCHEMA(’#{schema_name}‘, NULL, ?, ?)“
stmt = IBM_DB::prepare(conn, sql)
IBM_DB::bind_param(stmt, 1, “errortabschema“, IBM_DB::SQL_PARAM_INPUT)
IBM_DB::bind_param(stmt, 2, “errortab“, IBM_DB::SQL_PARAM_INPUT)
IBM_DB::execute(stmt)
end
ActiveRecord::Base.connection.execute(”DROP TABLESPACE SYSTOOLSPACE“) unless systool_existing
ActiveRecord::Base.connection.execute(”DROP SCHEMA ERRORSCHEMA RESTRICT“)
end
This can also be used do define a method on its own (e.g. purge_database(:mydb)), should you require that functionality somewhere in your code. In that case, just make sure to modify the first lines in order to use the already established connection or define your own.
rake db:test:clone
Now that rake db:test:purge is working, you will be able to successfully run rake db:test:clone. Out of the box, there are two limitations though. Rails doesn’t acknowledge tablespaces and foreign keys (the lack of the first is understandable as it is strongly related to DB2, but the lack of the second is much less justifiable).
This doesn’t affect many developers but if it does affect you, it is both annoying and problematic. Suppose in fact, that you have created the database objects in your development db through migrations. You may have specified a certain tablespace name by passing :options => “IN mytablespace” to the create_table method. Running rake db:test:clone will generate the table in the test database within the default tablespace (USERSPACE1) rather than in the one that you’ve specified for the development database. Not only this, but if you manually defined foreign keys by executing sql statements in the migrations or directly, these will not appear in the cloned test database.
This is not an issue specific to DB2, it is just the way it works in Rails at the moment. In fact there are third party plugins that attempt to introduce these and other features that are shortcoming of Rails’ core (e.g. Foreign Key Schema Dumper Plugin for MySQL and PostgreSQL).
In order to address those two concerns when using DB2, you don’t have to operate on the definition of the task directly. In fact, the db:test:clone loads the dumped schema in the test environment. The culprit is therefore ActiveRecord SchemaDumper which doesn’t know anything about DB2 Tablespaces and about foreign keys. Changes to this will also affect the db:schema:dump task, which will in turn produce more correct and “database aware” db\schema.rb files.
The file schema_dumper.rb within the ActiveRecord gem (activerecord-1.15.3\lib\active_record\schema_dumper.rb) can be directly modified for your specific needs. At line 21 the dump method becomes:
def dump(stream)
header(stream)
tables(stream)
if @connection.adapter_name == “IBM_DB“
foreign_keys(stream)
end
trailer(stream)
stream
end
At line 89 within the table method, we need to specify code to handle the possibility of a non-default tablespace:
if @connection.adapter_name == “IBM_DB“
tbspace_sql = “select TBSPACE from syscat.tables where tabname=’#{table.upcase}‘“
table_space = @connection.select_one(tbspace_sql)[”tbspace“]
if table_space != “USERSPACE1“
tbl.print %Q(, :options => “IN #{table_space}“)
end
end
At this point, just after the index method, we need to define the method foreign_keys:
def foreign_keys(stream)
references = @connection.select_all(”SELECT * FROM SYSCAT.REFERENCES“)
for reference in references
constraint = reference[”constname“]
schema = reference[”tabschema“]
table = reference[”tabname“]
cols = reference[”fk_colnames“]
ref_schema = reference[”reftabschema“]
ref_table = reference[”reftabname“]
ref_cols = reference[”pk_colnames“]
if reference[”updaterule“] == “R“
update_action = “RESTRICT“
else
update_action = “NO ACTION“
end
delete_action = case reference[”deleterule“]
when “A“
“NO ACTION“
when “R“
“RESTRICT“
when “C“
“CASCADE“
when “N“
“SET NULL“
end
foreign_key_sql = “ execute(“ALTER TABLE #{schema}.#{table} add
CONSTRAINT #{constraint} FOREIGN KEY (#{cols.strip})n
REFERENCES #{ref_schema}.#{ref_table} (#{ref_cols.strip})
ON UPDATE #{update_action} ON DELETE #{delete_action}n
ENFORCED ENABLE QUERY OPTIMIZATION“)n “
stream.print foreign_key_sql
stream.puts
end
end
As you can imagine, it is possible to prevent the need to modify the file directly, by simply extending the SchemaDumper class in, for example, a plugin. You would have to overwrite the original methods within the SchemaDumper class:
module ActiveRecord
class SchemaDumper
def dump(stream)
end
private
def table(table, stream)
end
def foreign_keys(stream)
end
end
end
It would be beneficial to aggregate several improvements in a “DB2 PowerPack” plugin of some sort, and it’s very likely that we will eventually work on publishing something like this.
June 20 2007 | How-to | 1 Comment »
The Rails adapter and the Ruby driver for IBM DB2 enable you to take full advantage of the exclusive pureXML capabilities. XML fields are properly handled and also recognized when using migrations. We do not bind the content of XML columns to a specific XML Ruby representation (e.g. REXML) but rather let the user decide what they’d prefer to use, in light also of the fact that mapping to a simple string is often all that is required given the fast XQuery/XPath querying features provided out of the box by DB2.
The scaffolding generator ignores XML fields though. This is due to the fact that the bult-in scaffolding generates form elements for only a few datatypes. For example :time and :binary are excluded, and foreign keys are not handled as well. Users will typically want to handle XML fields in a customized way, for instance, showing only certain elements of the XML document in their forms. However if you wish to enable by default the automatic generation of text area boxes when using scaffolding, you can edit C:\ruby\lib\ruby\gems\1.8\gems\actionpack-1.12.5\lib\action_view\helpers\active _record_helper.rb (or the equivalent on your system) by replacing the to_tag method with the following:
def to_tag(options = {})
case column_type
when :string
field_type = @method_name.include?(”password”) ? “password” : “text”
to_input_field_tag(field_type, options)
when :text, :xml
to_text_area_tag(options)
when :integer, :float
to_input_field_tag(”text”, options)
when :date
to_date_select_tag(options)
when :datetime, :timestamp
to_datetime_select_tag(options)
when :boolean
to_boolean_select_tag(options)
end
end
As you can see, all we are doing here is adding the XML datatype to the list of cases which require the rendering of a text area box.
January 06 2007 | How-to and Tips&Tricks | 1 Comment »
on Rails