+-------------+    +-------------+    +-------------+
   | Document    |    | Linkage     |    | Animal      |
   +-------------+    +-------------+    +-------------+
   | P1 id       |    | P1 web_page |    | P1 id       |
   |    data     |    | P1 animal   |    | P2 name     |
   |             |    |    data?    |    |    data     |
   +-------------+    +-------------+    +-------------+

P - Primary key (Unique, Not NULL)