var id = 1;
var query = database.Posts // your starting point - table in the "from" statement
.Join(database.Post_Metas, // the source table of the inner join
post => post.ID, // Select the primary key (the first part of the "on" clause in an sql "join" statement)
meta => meta.Post_ID, // Select the foreign key (the second part of the "on" clause)
(post, meta) => new { Post = post, Meta = meta }) // selection
.Where(postAndMeta => postAndMeta.Post.ID == id); // where statement
// for comparison
var id = 1;
var query =
from post in database.Posts
join meta in database.Post_Metas on post.ID equals meta.Post_ID
where post.ID == id
select new { Post = post, Meta = meta };