# reflection.rdoc

Path: | doc/reflection.rdoc
Last Update: | Thu Mar 18 05:47:28 -0600 2010

# Reflection

Sequel supports reflection information in multiple ways.

## Adapter in Use

You can get the adapter in use using Database.adapter_scheme. As this is a class method, you generally need to do DB.class.adapter_scheme:

      DB.class.adapter_scheme # e.g. :postgres, :jdbc, :odbc

## Database Connected To

In some cases, the adapter scheme will be the same as the database to which you are connecting. However, many adapters support multiple databases. You can use the Database#database_type method to get the type of database to which you are connecting:

      DB.database_type # :postgres, :h2, :mssql

## Tables in the Database

On many database types/adapters, Database#tables exists and gives an array of table name symbols:

      DB.tables # [:table1, :table2, :table3, ...]

## Indexes on a table

On a few database types/adapters, Database#indexes takes a table name gives a hash of index information. Keys are index names, values are subhashes with the keys :columns and :unique :

      DB.indexes(:table1) # {:index1=>{:columns=>[:column1], :unique=>false}, :index2=>{:columns=>[:column2, :column3], :unique=>true}}

Index information generally does not include partial indexes, functional indexes, or indexes on the primary key of the table.

## Column Information for a Table

Database#schema takes a table symbol and returns column information in an array with each element being an array with two elements. The first elements of the subarray is a column symbol, and the second element is a hash of information about that column. The hash should include the following keys:

- :allow_null - Whether NULL/nil is an allowed value for this column. Used by the Sequel::Model typecasting code.
- :db_type - The type of column the database provided, as a string. Used by the schema_dumper plugin for a more specific type translation.
- :default - The default value of the column, as either a string or nil. Uses a database specific format. Used by the schema_dumper plugin for converting to a ruby value.
- :primary_key - Whether this column is one of the primary key columns for the table. Used by the Sequel::Model code to determine primary key columns.
- :ruby_default - The default value of the column as a ruby object, or nil if there is no default or the default could not be successfully parsed into a ruby object.
- :type - The type of column, as a symbol (e.g. :string). Used by the Sequel::Model typecasting code.

Example:

      DB.schema(:table) # [[:column1, {:allow_null=>true, :db_type=>'varchar(255)', :default=>'blah', :primary_key=>false, :type=>:string}], ...]

## Column Information for a Model

Model#db_schema returns pretty much the same information, except it returns it as a hash with column keys instead of an array of two element arrays.

      Model.db_schema # {:column1=>{:allow_null=>true, :db_type=>'varchar(255)', :default=>'blah', :primary_key=>false, :type=>:string}, ...}

## Columns used by a dataset/model

Dataset#columns returns the columns of the current dataset as an array of symbols:

      DB[:table].columns # [:column1, :column2, :column3, ...]

Dataset#columns! does the same thing, except it ignores any cached value. In general, the cached value should never be incorrect, unless the database schema is changed after the dataset is created.

      DB[:table].columns! # [:column1, :column2, :column3, ...]

Model.columns does the same thing as Dataset#columns, using the model‘s dataset:

      Model.columns # [:column1, :column2, :column3, ...]

## Associations Defined

Sequel::Model offers complete introspection capability for all associations.

You can get an array of association symbols with Model.associations:

      Model.associations # [:association1, :association2, ...]

You can get the association reflection for a single association via the Model.association_reflection. Association reflections are subclasses of hash, for ease of use and introspection (and backwards compatibility):

      Model.association_reflection(:association1) # {:name=>:association1, :type=>:many_to_one, :model=>Model, :associated_class=>OtherModel, ...}

You can get an array of all association reflections via Model.all_association_reflections:

      Model.all_association_reflections # [{:name=>:association1, :type=>:many_to_one, :model=>Model, :associated_class=>OtherModel, ...}, ...]

Finally, you can get a hash of association reflections via Model.association_reflections:

      Model.association_reflections # {:association1=>{:name=>:association1, :type=>:many_to_one, :model=>Model, :associated_class=>OtherModel, ...}, ...}

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