Example showing how to use Sequel without a singleton database. It uses two DB connections -- one that represents a sharded connections (e.g. to connect to many DBs with the same schema) and then a second connection to an individual database with a different schema.

sequel_example_spec.rb

require 'sequel/model'
Sequel.extension :arbitrary_servers, :server_block

non_sharded = Sequel.connect \
  adapter:  'mysql2',
  host:     "localhost",
  database: "vanguard_non_sharded",
  user:     'root',
  password: nil

NonShardedModel = Sequel::Model(non_sharded)

sharded = Sequel.connect \
  adapter:  'mysql2',
  user:     'root',
  password: nil,
  servers:  { default: proc { raise "No current shard" } }

sharded.extend Sequel::ServerBlock
sharded.pool.extend Sequel::ArbitraryServers

# ensure we synchronize DB access.
# see: https://github.com/jeremyevans/sequel/blob/540d336c5845cb007b32234edbbfd1cc5462d527/doc/sharding.rdoc
def sharded.with_server(*args)
  super(*args) { synchronize { |c| yield c } }
end

def sharded.use(host, database)
  with_server(host: host, database: database) do
    yield self
  end
end

ShardedModel = Sequel::Model(sharded)
def ShardedModel.set_dataset(*args)
  # Sequel needs a valid DB connection when setting the dataset,
  # because it makes a query to get the schema in order to
  # determine the columns.
  db.use('localhost', 'master_schema_db') { super }
end

class ShardBuild < NonShardedModel
end

class Keyword < ShardedModel
end

describe ShardBuild do
  it 'has the expected columns' do
    expect(ShardBuild.columns).to include(:shard_num, :campaign_id)
  end
end

describe Keyword do
  around(:each) do |example|
    sharded.use("localhost", "my_test_db") do |db|
      db.transaction(rollback: :always, &example)
    end
  end

  it 'has the expected columns' do
    expect(Keyword.columns).to include(:id, :keyword)
  end

  it 'can insert and query data' do
    Keyword.dataset.insert(keyword: "foo", id: 15, branding: "auto", created_at: Time.now)
    expect(Keyword.where(:keyword => "foo").count).to eq(1)
  end
end