Find object by attribute - ruby

Question

I have a ruby class like this:
class Table
def initialize(name)
#name = name
#columns = {}
end
end
I'm creating different objects:
table_1 = Table.new("First")
table_2 = Table.new("Second")
table_3 = Table.new("Third")
How can I find among the objects of the Table class the object having "Second" as name attribute ?

Answer 1

Without creating any additional structures of data:
ObjectSpace.each_object(Table).find { |object| object.instance_variable_get(:#name) == "Second" }
=> #<Table:0x007f9f912b0ce0 #columns={}, #name="Second">

Answer 2

You can keep a reference to an array of instances in the class.
class Table
#instances = []
class << self
attr_accessor :instances
end
def initialize(name)
#name = name
#columns = {}
self.class.instances << self
end
end
Then you can get all the instances by
Table.instances
This, however, will prevent all the Table objects being garbage collected, so it is only feasible if you have only a small amount of Tables and that amount never grows, otherwise you'll have a memory leak.

Answer 3

Let's add a getter method for the name attribute
class Table
attr_reader :name
def initialize(name)
#name = name
#columns = {}
end
end
Now, if you have an array of Table objects
arr = [Table.new("First"), Table.new("Second"), Table.new("Third")]
You can find by name
arr.find { |table| table.name == "Second" }
=> #<Table:0x007f862107eb18 #name="Second", #columns={}>

Answer 4

You can use enumerable find or any similar method from the enumerable module:
class Table
attr_reader :name
def initialize(name)
#name = name
#columns = {}
end
end
table_1 = Table.new("First")
table_2 = Table.new("Second")
table_3 = Table.new("Third")
x = [table_1, table_2, table_3].find { |t| t.name == "Second" }
puts x.name => "Second"

Answer 5

Let's say we have reader on name in Table:
class Table
attr_reader :name
def initialize(name)
#name = name
#columns = {}
end
end
I'd encourage you to store these Table classes' objects in a list/array:
table_1 = Table.new("First")
table_2 = Table.new("Second")
table_3 = Table.new("Third")
tables = [table_1, table_2, table_3]
Which can then be used for finding it using find(as mentioned in one of the answers) or detect:
tables.detect { |t| t.name == "Second" } #=> table_2 object
If you'd like to go one more step ahead then we can have another class maintaining this array:
class TableList
attr_reader :tables
def initialize
#tables = tables
end
def add(table)
#tables << table
end
def find_by_name(name)
tables.detect{ |table| table.name == name }
end
end
Which can then be used as:
table_1 = Table.new("First")
table_2 = Table.new("Second")
table_3 = Table.new("Third")
table_list = TableList.new
table_list.add(table_1)
table_list.add(table_2)
table_list.add(table_3)
table_list.find_by_name('Second') #=> table_2 object