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5612 arel table
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For cases in which you want to generate custom SQL yourself through Arel, you may use the arel_table method to gain access to the Table for the class
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>> users = Userarel_table >> userswhere(users[:login]eq("mack"))to_sql => "SELECT 'users''id', 'users''login' FROM 'users' WHERE 'users''login' = 'mack'"
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You can consult the Arel documentation directly on how to construct custom queries using its DSL5
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5 http://githubcom/rails/arel/
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57 Connections to Multiple Databases in Different Models
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57 Connections to Multiple Databases in Different Models
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Connections are created via ActiveRecord::Baseestablish_connection and retrieved by ActiveRecord::Baseconnection All classes inheriting from ActiveRecord::Base will use this connection What if you want some of your models to use a different connection You can add class-specific connections For example, let s say you need to access data residing in a legacy database apart from the database used by the rest of your Rails application We ll create a new base class that can be used by models that access legacy data Begin by adding details for the additional database under its own key in databaseyml Then call establish_connection to make LegacyProjectBase and all its subclasses use the alternate connection instead
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class LegacyProjectBase < ActiveRecord::Base establish_connection :legacy_database selfabstract_class = true end
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Incidentally, to make this example work with subclasses, you must specify selfabstract_class = true in the class context Otherwise, Rails considers the subclasses of LegacyProject to be using single-table inheritance (STI), which we discuss at length in 9 Xavier says
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You can easily point your base class to different databases depending on the Rails environment like this:
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class LegacyProjectBase < ActiveRecord::Base establish_connection "legacy_#{Railsenv}" selfabstract_class = true end
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Then just add multiple entries to databaseyml to match the resulting connection names In the case of our example, legacy_development, legacy_test, etc
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The establish_connection method takes a string (or symbol) key pointing to a configuration already defined in databaseyml Alternatively, you can pass it a literal hash of options, although it s messy to put this sort of configuration data right into your model file instead of databaseyml
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class TempProject < ActiveRecord::Base establish_connection :adapter => 'sqlite3', :database => ':memory:' end
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Rails keeps database connections in a connection pool inside the ActiveRecord::
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Base class instance The connection pool is simply a Hash object indexed
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by Active Record class During execution, when a connection is needed, the
retrieve_connection method walks up the class-hierarchy until a matching con-
nection is found
58 Using the Database Connection Directly
It is possible to use Active Record s underlying database connections directly, and sometimes it is useful to do so from custom scripts and for one-off or ad-hoc testing Access the connection via the connection attribute of any Active Record class If all your models use the same connection, then use the connection attribute of ActiveRecord::Base
ActiveRecord::Baseconnectionexecute("show tables")all_hashes
The most basic operation that you can do with a connection is simply to execute a SQL statement from the DatabaseStatements module For example, Listing 51 shows a method that executes a SQL file statement by statement
Listing 51 Execute a SQL file line by line using active record s connection def execute_sql_file(path) Fileread(path)split(';')each do |sql| begin ActiveRecord::Baseconnectionexecute(#{sql}\n") unless sqlblank rescue ActiveRecord::StatementInvalid $stderrputs "warning: #{$!}" end end end
581 The DatabaseStatements Module
The ActiveRecord::ConnectionAdapters::DatabaseStatements module mixes a number of useful methods into the connection object that make it possible to work with the database directly instead of using Active Record models I ve purposely left out some of the methods of this module (such as add_limit! and add_lock) because they
58 Using the Database Connection Directly
are used internally by Rails to construct SQL statements dynamically and I don t think they re of much use to application developers For the sake of readability in the select_examples below, assume that the connection object has been assigned to conn, like this:
conn = ActiveRecord::Baseconnection
begin db transaction()
Begins a database transaction manually (and turns off Active Record s default autocommitting behavior)
commit db transaction()
Commits the transaction (and turns on Active Record s default autocommitting behavior again)
delete(sql statement)
Executes a SQL DELETE statement provided and returns the number of rows affected
Active Record
execute(sql statement)
Executes the SQL statement provided in the context of this connection This method is abstract in the DatabaseStatements module and is overridden by specific database adapter implementations As such, the return type is a result set object corresponding to the adapter in use
insert(sql statement)