Fixed some issues with the DB initializer and added some stuff to README

README.md

@@ -59,6 +59,26 @@ row.commit(transaction)
 row.sync # true if synchronization successful
 ```
 
+### Explicitly request an existing row or explicitly creating a new row
+
+```ruby
+#...
+row = Table.create({"pk1" => "new", "pk2" => "something", ...}) # returns nil if row exists
+
+row = Table.get({"pk1" => "new", "pk2" => "something", ...}) # returns nil if row does not exist
+```
+
+### Mapping results of a query to objects of a table
+
+```ruby
+rows = Table.map("SELECT * FROM table_name WHERE row > 4 # AND ...")
+pp rows
+
+#=>[<Table:0x000055da208232c8 @data={"row" => 5, ...}>,
+#   <Table:0x00001238989cc89d @data={"row" => 7, ...}>,
+#   ...]
+```
+
 And that's really about it.
 
 ## Installation

lib/lpgar.rb

@@ -132,8 +132,9 @@ module LPGAR
         @sync_query = <<~QUERY
           SELECT * FROM #{@table_name}
           WHERE #{selector}
-          LIMIT 1;
+          LIMIT 1
         QUERY
+        puts @sync_query
       end
 
       # Returns transaction class for this record.
@@ -363,10 +364,14 @@ module LPGAR
         @cols = conn.exec_prepared('lpgar_get_columns', [name]).map do |row|
           row['column_name']
         end
+        raise StandardError, "table #{name} doesn't exist" if @cols.empty?
+
         @cols_pk = conn.exec_prepared('lpgar_get_pk', [name]).map do |row|
           row['attname']
         end
-        raise StandardError, "table #{name} doesn't exist" if @cols.empty?
+        if @cols_pk.empty?
+          raise StandardError, "table #{name} has no primary keys"
+        end
 
         @conn = conn
         @instances = {}