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Include this module in an attribute type to customize the SQL expression used to compare values of that type in Active Record predicates.

For example, a text type can compare through the normalized expression used by a database expression index:

def comparison_expression(expression)
  Arel::Nodes::NamedFunction.new("lower", [expression])
end

Active Record compares values of the type by applying the expression symmetrically to the stored attribute and query value. Ruby values are serialized first; Arel expressions are not. IS NULL tests apply to the transformed attribute, and subquery projections are treated as stored values and transformed as well. Association join constraints compare two stored attributes. Each side is transformed through its own column’s type. The expression defines equality, ordering, and null semantics for the type. Type authors are responsible for ensuring those semantics are appropriate and supported by the database.

Ordering compares through the same expression: order on an attribute sorts by the transformed value, as do in_order_of and batch cursors. Writes use serialize without applying the comparison expression; group and aggregate calculations use the stored attribute.

Namespace
Methods
C

Instance Public methods

comparison_expression(expression)

Converts a stored attribute or serialized query value into the expression on which comparisons are performed.

Source: show | on GitHub 

# File activerecord/lib/active_record/type/query_predicates.rb, line 37
def comparison_expression(expression)
  expression
end