Last Update: 2017-08-01 08:12:00 -0700


Sequel supports reflection information in multiple ways.

Adapter in Use

You can get the adapter in use using Database#adapter_scheme:

DB.adapter_scheme # :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

Database#tables gives an array of table name symbols:

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

Views in the Database

Database#views and gives an array of view name symbols:

DB.views # [:view1, :view2, :view3, ...]

Indexes on a table

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.

Foreign Key Information for a Table

Database#foreign_key_list takes a table name and gives an array of hashes of foreign key information:

DB.foreign_key_list(:table1) # [{:columns=>[:column1], :table=>:referenced_table, :key=>[:referenced_column1]}]

At least the following entries will be present in the hash:


An array of columns in the given table


The table referenced by the columns


An array of columns referenced (in the table specified by :table), but can be nil on certain adapters if the primary key is referenced.

The hash may also contain entries for:


Whether the constraint is deferrable


The name of the constraint


The action to take ON DELETE


The action to take ON UPDATE

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:


Whether NULL/nil is an allowed value for this column. Used by the Sequel::Model typecasting code.


The type of column the database provided, as a string. Used by the schema_dumper plugin for a more specific type translation.


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.


Whether this column is one of the primary key columns for the table. Used by the Sequel::Model code to determine primary key columns.


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.


The type of column, as a symbol (e.g. :string). Used by the Sequel::Model typecasting code.


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:

Model.association_reflection(:association1) # #<Sequel::Model::Associations::ManyToOneAssociationReflection Model.many_to_one :association1>

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

Model.all_association_reflections # [#<Sequel::Model::Associations::ManyToOneAssociationReflection Model.many_to_one :association1>, ...]

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

Model.association_reflections # {:association1=>#<Sequel::Model::Associations::ManyToOneAssociationReflection Model.many_to_one :association1>, ...}

Validations Defined

When using the validation_class_methods plugin, you can use the validation_reflections class method to get a hash with validation reflection information. This returns a hash keyed on the column name symbol:

Model.validation_reflections[:column] # => [[:presence, {}], [:length, {:maximum=>255, :message=>'is just too long'}]]

Similarly, when using the constraint_validations plugin, you can use the constraint_validation_reflections class method:

Model.constraint_validation_reflections[:column] # => [[:presence, {}], [:max_length, {:argument=>255, :message=>'is just too long'}]]