[ENSCORESW-694]. Adding a unique constraint to alt_allele(gene_id).

Seems that our earlier design was incorrect and we do want to only
have 1 gene in 1 alt allele group. So adding the constraint in.

sql/patch_73_74_h.sql

+# patch_73_74_h.sql
+#
+# Title: Creating a unique index on alt_allele(gene_id)
+#
+# Description:
+#
+# Alt alleles only allow a gene to appear in one group. Enforce this 
+# at the DB level
+
+CREATE UNIQUE INDEX gene_idx ON alt_allele(gene_id);
+
+# patch identifier
+INSERT INTO meta (species_id, meta_key, meta_value) VALUES (NULL, 'patch', 'patch_73_74_h.sql|alt_allele_unique_gene_idx');
+
+ 

sql/table.sql

@@ -274,6 +274,8 @@ INSERT INTO meta (species_id, meta_key, meta_value)
  VALUES (NULL, 'patch', 'patch_73_74_f.sql|remove_pair_dna_align');
 INSERT INTO meta (species_id, meta_key, meta_value) 
   VALUES (NULL, 'patch', 'patch_73_74_g.sql|add_transcript_idx_tise');
+INSERT INTO meta (species_id, meta_key, meta_value) 
+  VALUES (NULL, 'patch', 'patch_73_74_h.sql|alt_allele_unique_gene_idx');
 
 /**
 @table meta_coord
@@ -405,6 +407,7 @@ CREATE TABLE alt_allele (
         gene_id INT UNSIGNED NOT NULL,
 
         PRIMARY KEY (alt_allele_id),
+        UNIQUE KEY gene_idx (gene_id),
         KEY (gene_id,alt_allele_group_id)
 
 ) COLLATE=latin1_swedish_ci ENGINE=MyISAM;