# File activerecord/lib/active_record/connection_handling.rb, line 187
def clear_query_caches_for_current_thread
  ActiveRecord::Base.connection_handlers.each_value do |handler|
    handler.connection_pool_list.each do |pool|
      pool.connection.clear_query_cache if pool.active_connection?
    end
  end
end

Clears the query cache for all connections associated with the current thread.

# File activerecord/lib/active_record/connection_handling.rb, line 235
def connected?
  connection_handler.connected?(connection_specification_name)
end

Returns true if Active Record is connected.

# File activerecord/lib/active_record/connection_handling.rb, line 117
def connected_to(database: nil, role: nil, &blk)
  if database && role
    raise ArgumentError, "connected_to can only accept a `database` or a `role` argument, but not both arguments."
  elsif database
    if database.is_a?(Hash)
      role, database = database.first
      role = role.to_sym
    end

    config_hash = resolve_config_for_connection(database)
    handler = lookup_connection_handler(role)

    handler.establish_connection(config_hash)

    with_handler(role, &blk)
  elsif role
    with_handler(role.to_sym, &blk)
  else
    raise ArgumentError, "must provide a `database` or a `role`."
  end
end

Connects to a database or role (ex writing, reading, or another custom role) for the duration of the block.

If a role is passed, Active Record will look up the connection based on the requested role:

ActiveRecord::Base.connected_to(role: :writing) do
  Dog.create! # creates dog using dog writing connection
end

ActiveRecord::Base.connected_to(role: :reading) do
  Dog.create! # throws exception because we're on a replica
end

ActiveRecord::Base.connected_to(role: :unknown_role) do
  # raises exception due to non-existent role
end

For cases where you may want to connect to a database outside of the model, you can use connected_to with a database argument. The database argument expects a symbol that corresponds to the database key in your config.

ActiveRecord::Base.connected_to(database: :animals_slow_replica) do
  Dog.run_a_long_query # runs a long query while connected to the +animals_slow_replica+
end

This will connect to a new database for the queries inside the block. By default the `:writing` role will be used since all connections must be assigned a role. If you would like to use a different role you can pass a hash to database:

ActiveRecord::Base.connected_to(database: { readonly_slow: :animals_slow_replica }) do
  # runs a long query while connected to the +animals_slow_replica+ using the readonly_slow role.
  Dog.run_a_long_query
end

When using the database key a new connection will be established every time.

# File activerecord/lib/active_record/connection_handling.rb, line 145
def connected_to?(role:)
  current_role == role.to_sym
end

Returns true if role is the current connected role.

ActiveRecord::Base.connected_to(role: :writing) do
  ActiveRecord::Base.connected_to?(role: :writing) #=> true
  ActiveRecord::Base.connected_to?(role: :reading) #=> false
end

# File activerecord/lib/active_record/connection_handling.rb, line 198
def connection
  retrieve_connection
end

Returns the connection currently associated with the class. This can also be used to “borrow” the connection to do database work unrelated to any of the specific Active Records.

# File activerecord/lib/active_record/connection_handling.rb, line 222
def connection_config
  connection_pool.spec.config
end

Returns the configuration of the associated connection as a hash:

ActiveRecord::Base.connection_config
# => {pool: 5, timeout: 5000, database: "db/development.sqlite3", adapter: "sqlite3"}

Please use only for reading.

# File activerecord/lib/active_record/connection_handling.rb, line 205
def connection_specification_name
  if !defined?(@connection_specification_name) || @connection_specification_name.nil?
    return primary_class? ? "primary" : superclass.connection_specification_name
  end
  @connection_specification_name
end

Return the specification name from the current class or its parent.

# File activerecord/lib/active_record/connection_handling.rb, line 68
def connects_to(database: {})
  connections = []

  database.each do |role, database_key|
    config_hash = resolve_config_for_connection(database_key)
    handler = lookup_connection_handler(role.to_sym)

    connections << handler.establish_connection(config_hash)
  end

  connections
end

Connects a model to the databases specified. The database keyword takes a hash consisting of a role and a database_key.

This will create a connection handler for switching between connections, look up the config hash using the database_key and finally establishes a connection to that config.

class AnimalsModel < ApplicationRecord
  self.abstract_class = true

  connects_to database: { writing: :primary, reading: :primary_replica }
end

Returns an array of established connections.

# File activerecord/lib/active_record/connection_handling.rb, line 158
def current_role
  connection_handlers.key(connection_handler)
end

Returns the symbol representing the current connected role.

ActiveRecord::Base.connected_to(role: :writing) do
  ActiveRecord::Base.current_role #=> :writing
end

ActiveRecord::Base.connected_to(role: :reading) do
  ActiveRecord::Base.current_role #=> :reading
end

# File activerecord/lib/active_record/connection_handling.rb, line 49
def establish_connection(config_or_env = nil)
  config_hash = resolve_config_for_connection(config_or_env)
  connection_handler.establish_connection(config_hash)
end

Establishes the connection to the database. Accepts a hash as input where the :adapter key must be specified with the name of a database adapter (in lower-case) example for regular databases (MySQL, PostgreSQL, etc):

ActiveRecord::Base.establish_connection(
  adapter:  "mysql2",
  host:     "localhost",
  username: "myuser",
  password: "mypass",
  database: "somedatabase"
)

Example for SQLite database:

ActiveRecord::Base.establish_connection(
  adapter:  "sqlite3",
  database: "path/to/dbfile"
)

Also accepts keys as strings (for parsing from YAML for example):

ActiveRecord::Base.establish_connection(
  "adapter"  => "sqlite3",
  "database" => "path/to/dbfile"
)

Or a URL:

ActiveRecord::Base.establish_connection(
  "postgres://myuser:mypass@localhost/somedatabase"
)

In case ActiveRecord::Base.configurations is set (Rails automatically loads the contents of config/database.yml into it), a symbol can also be given as argument, representing a key in the configuration hash:

ActiveRecord::Base.establish_connection(:production)

The exceptions AdapterNotSpecified, AdapterNotFound and ArgumentError may be returned on an error.